i 



BRITISH FOSSILS. 



ambuiacral margin. If this variety had a truncated posterior 

 margin it would be the P. Morrisii, Wright, a very closely allied 

 species. In others the form is slightly more pentagonal, and in 

 one extreme variety the form is as angular and the sides of the 

 pyramid as flat, as in the species next to be described (P. conoideus). 

 But the tubercles are large, not minute, and the ambulacra though 

 elevated have not sunk avenues, as in that species. The base 

 occasionally is not flat, but undulated, the interambulacra being- 

 tumid on the lower surface. 



The variation observable in the young state, viz., that the anal 

 opening is very small and narrow, has before been adverted to. 

 Only the two primary rows of tubercles in each area show distinctly 

 when the specimens are but half an inch across. 



Affinities. — "P. semisulcatiis most nearly resembles P. um- 

 brella.^ We have before us specimens which are well represented by 

 Agassiz s figure, but the loss of the tubercles from the Swiss spe- 

 cimen leaves the question of their identity an open one.'' (Wright.) 

 And we may add, that in the Swiss species the anal opening is 

 larger and more pyriform, and the tumid ridge on either side of it is 

 absent. But it appears to be present, to judge from the profile, in 

 F.patelliformis, figured byAgassiz in the same plate (Ech. Suisses, 

 tab. 13, f 1-8.), a species in which we can scarcely see any character 

 which should sufiiciently separate it from P. semisulcatus, unless 

 it be the breadth of the ambulacra and the continuation of their 

 inner rows of tubercles higher up than in ours. The anus is cer- 

 tainl)^ larger and seems to be pyriform rather than oval- oblong. 

 This last difference may be sufficient to distinguish the two species, 

 and it is accordingly inserted in the specific character. P. laganoides, 

 Agass., figured in the same work, and in Desor's Monograph, is even 

 more like, especially in the form of the anus, which is also placed 

 in a depression along with the plates of the disk. The greater 

 breadth of the ambulacra, the larger size of the mouth, and the 

 much closer and more regularly placed tubercles, however, prevent 

 us from uniting it with ours. P. tenuis of Desor's Monograph, 

 tab. 12. f 1-3., is so very like in the arrangement of the tubercles, 

 nan-ow ambulacra, and sunk mouth, that but for the more depressed 

 shape and large anal opening, which too, is not flanked by tumid 

 ridges, it might be identical. It is from the J ura, in the " Terrain 

 a Chailles." 



History. — The list of synonyms was all that was left regarding 

 this species by Professor Forbes, and in deference to him the 

 * Now P. dilatatus, Ag., Cat. Eaisonne, 1. c, p. 144. 



