342 PSEUDODIADEMA. 



and are througliout 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. G, 11) seems to 

 have had ten tubercles in the inner rows. The miliary zone is narrow in the infra- 

 marginal 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, though 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 

 PI. XIV, fig. 2 c. 



Locality and Stratiyraphical 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, however, were not wanting to show 

 that the fracture of the piece of chalk containing the Pseudodiadema had split the test into 

 two halves, along the plane of the ambitus, and that the second piece of chalk which 

 bore the impression of these plates (and which fortunately had been saved) contained the 

 under plates of the Urchin. The two halves Avere afterwards mounted on plaster of 

 Paris, and carefully cleaned until the parts of the test previously concealed began to 

 appear. In the removal of the chalk evidence was given that several extremely fine 

 hair-like spines, with a striated and quasi-imbricated exterior, were in contact witti or 

 close to the test, and were the spines of the Urchin. Such spines are occasionally met 

 with in the Upper Chalk, and solitary plates similar to those depicted in figs. 1, 5, are 

 found in the same geological horizon. It is very unusual for the plates and spines to be 

 associated together as in the present instance. 



Affinities and Differences. — Pseudodiadema fragile has some resemblance to Pseudo- 

 diadema ornatum of the Lower Chalk, but can be distinguished by its smaller and 

 widely separated tubercles, the general smooth surface of its test, and its spines with their 

 series of short longitudinal striations, which by their divergence form a succession of 

 fringes. The circumstance that the plates of this Urchin are generally found separated 

 from one another has suggested the specific name oi fragile.'" 



At pages 1 — 14 I gave an account of the subdivisions of the Cretaceous beds of 

 England as they were recognised at that period when that part of my ]\Ionograph was 



