Diadematidae

Gray, 1855

Genre type : Diadema Gray, 1825, p.426

Syn : Centrechinidae Jackson, 1912 ; Lissodiadematidae Fell, 1966

Description succinte de la famille : Apex hemicyclique, occulaires postérieures insertes. Les plaques génitales rentrent loin dans l'inter-radius. Tubercules perforés, crénelés (presque toujours). Radioles verticillés.

 

 


Genre Centrostephanus  Peters, 1855

Espèce type  Diadema longispina PhIlippi,1845

Extension stratigraphique (bibliographique, non vérifiée) : Santonien - Actuel

Syn.

  • Thrichodiadema Agassiz, 1863, p.354. Espèce type T. rodgersii Agassiz ; synonyme subjectif plus récent

  • Echinodiadema Verrill, 1867. Espèce type E. coronata Verrill, 1867, p.580 ; synonyme subjectif plus récent

  • Palaeodiadema Pomel, 1887, p.318. Espèce type P. fragile Wiltshire in Wright, 1882, p.341 ; synonyme subjectif plus récent

  • Helikodiadema Gregory, 1896. Espèce type P. fragile  Verrill, 1867, Wiltshire in Wright, 1882, p.341 ; synonyme subjectif plus récent

 
 

Centrostephanus fragilis (Wiltshire in Wright,1882)

 
 
diagnose de l'espèce par Wiltshire
Monograph of the british fossil echinodermata from the cretaceous formations vol I, (in Wright, 1882, p.341
 

     The following description and figures of a new species of Pseudodiadema ha ve been forwarded to me by my valued friend, the Rev. Professor Thos. Wiltshire, F.G.S., as a contribution to my Addenda.

Pseudodiadema fragile, Wiltshire. Pl. LXXX, figs. 1-11.

     "Test somewhat small, thin, circular, depressed, almost flat on upper surface, slightly convex on under. Poriferous zones narrow, straight, pores unigeminal on upper surface, trigeminal on lower. Ambulacra narrow above, rather wider below, with two rows of small perforated tubercles fromoral aperture to above ambitus. Interambulacral wide, two rows of small primary perforated tubercles in the middle extending the whole distance, and two rows of secondary perforated tubercles on lower side, ceasing after passing the ambitus. Miliary zone granular below, showing a few secondary tubercles above and below the ambitus. Mouth-opening large, peristome circular, lobed. Discal opening large, pentagular. Spines small, slender, straight ; surface striated, striae interrumted and diverging, and producing a roughened surface.

     Dimensions. - Height less than 1/2 inch, transverse diameter 1 1/2 inches.

     Description. - The test is thin, circular, almost flat on upper surface, slightly rounded on under. In the specimen from which the drawings were made the upper and under sides, prior to their fossilisation, had been subjected to considerable pressure, and brought into closer contact than they were in the living stage. The distance between the upper and the under sides therefore cannot be accurately determined ; probably the figured specimen was under half an inch in heigt.

     The ambulacral areas are straight and narrow (about two-ninths the width of the ambulacra at the upper side, and nearly one-half at the peristome), contracting in their range upwards. Two rows of about nine perforated tubercles (one tubercle to each plate) start from the under side, and cease soon after passing the ambitus. The tubercles are small, placed in small areolae, with slightly projecting bosses and crenulated summits, and are separated by an undulating series of granulations. The poriferous zones are narrow and straight. The pores are oval and simple, and are arranged in single file throughout the zones, save near the peristome, where they form into a few transverse lines of sets of three pores.

     The interambulacral areas are four times the width of the ambulacral at the equator, d are throughout much wider than the latter. On the under side they have four rows of small perforated primary tubercles, crenulated, and slightly raised above a smooth areola, whose margin is generally surrounded by granules. The two inner rows are best developed, and extend from the peristome to the discal margin ; the two exterior rows are absent from the uppermost plates. The figured specimen (figs. 6, 11) seems to have had ten tubercles in the inner rows. The miliary zone is narrow in the inframarginal region, is wider in the middle, and expands at the upper side ; it is filled with small irregularly placed granules, among which, on the upper side near the junction of plates, are a few mamillated tubercles, one on each plate.

 

     The spines (figs. 3, 8) are extremely fine, needle-shaped, rather longer than twice the width of the plates of the ambitus ; are solid, through occasionally longitudinal internal cavities exist. The surface is sculptured with fine long lines, and is marked by alternate angular contractions and expansions, so that the section lengthways is serrate, but circular in the opposite direction. A representation of a similar spine will be found in Pl. XIV, fig. 2 c.

     Locality and Stratigraphical Position.-The very rare Urchin illustrated on Pl.LXXX, figs. 1-11 was obtained many years since by myself from the Upper Chalk at Gravesend. When the fossil was first discovered it exhibited no more than the inner surface of the plates of the upper side. Indications, hewever, were not wanting to show that the fracture of the piece of chalk containing the Pseudodiadema hat split the test into two halves, along the plane of the ambitus, and that the second piece of chalk whchi bore the impression of these plates (and which fortunately had been saved) containded the under plates of the Urchin. The two halves were afterwards mounted on plaster of Paris, and carefully cleaned until the parts of the test presviously concealed began to appear. In the removal of the chalk evince was given that several extremely fine hair-like spines, with a striated and quasi-imbricated exterior, were in contact with or close to the test, and were the spines of the Urchin. Such spines are occasionally met with in the Upper Charlk, and solitary plates similar to those depicted in fig. 1, 5, are found in the same geological horizon. Itis very unusual for the plate and spines to be associated together as in the present instance.

      Affinities and Differences.-Pseuldodiadema fragile has some resemblance to Pseudodiadema ornatum of the Lower Chalk, but can be distinguished  by its smaller and widely separated tubercles, tu general smooth surface of its test, and its spines with their series of short longitudinal striations, which by their divergence for a succession of fringes. The circumstance that the plates of this Urchin are generally found separated from one another has suggested the specific name of fragile."

Extrait de la planche LXXX (figs. 1-11)

   
 
 

Centrostephanus fragilis (Wiltshire in Wright,1882) - Turonien sup, Seine-Maritime, 39 et 50 mm