436 PEor. p. M. DrNCAi?" o:s the sxErcTUPvE op 



Lamk. Desor notices that the amount of the cutting of the peristome 

 differs in the species, and that in some specimens of the typical 

 species this characteristic peculiaritj^ is not so intense as in others. 

 jSfevertheless Desor, with his usual sagacity, seized upon the cha- 

 racter T^hich of itself distinguished this genus from its fossil allies. 

 There are numerous species, and the specimens, so far as I can 

 make out, have some other distinctive structures equally important 

 with that chosen by the founder of the genus. These freshly noticed 

 characters, well known to all writers, are of great importance now 

 that the structure of the ambulacra is decided. Taking a good 

 specimen of Stomechinus higranularis from the Great Oolite, it is 

 seen that the apical disk is small and compact, the basal plates 

 project well into the median line of the interradia, and this line is 

 very bare and well marked by the vertical suture. The triangular 

 basals have large generative pores and alone form the anal ring. 

 The radials are wide at the adoral edge, which is notched, and the 

 optic pore is in that edge. 



The tubercles of both areas are smooth, imperforate, non-crenu- 

 lated, and not very unequal. The rows of tubercles diminish 

 in numbers abactinally. The ambulacra are about one half of the 

 width of the interradia above the ambitus, and at the peristome the 

 ambulacra are nearly twice as wide as the interradia. The pores are 

 in triplets, and above the ambitus the series of threes are very 

 oblique and barely in arcs, but nearer the peristome they are more 

 in arcs. The pairs are verj* numerous, and although there is a 

 crowding towards the peristome still the pairs are in place. 



Xear the radial plate the poriferous plates are low and broad 

 primaries. A little way down, and where the first large tubercle 

 is seen, three primaries have combined to form a compound plate, 

 low and broad. The adoral plate of the compound is a low 

 primary with a slight aboral bend, and the aboral plate is also low 

 and broad, and it has an adoral curve where it comes in contact 

 with the middle plate ; this is low at the poriferous part and 

 expanded towards and at the median line. There are no demi- 

 plates. The cuts are well developed, and yet in a form which will 

 for the future be accessible in the British Museum these branchial 

 shts are not so very distinctive ; but there is always the great width 

 of the ambulacra at the peristome and the remarkable diminution of 

 the size of the interradia there. 



It appears therefore that, bearing in mind the deep branchial cuts 

 of some modern Diadematidse ; and the fact that these and other 

 members of the family have bare median spaces, the peculiar shape 

 of the component plates of the compounds, and the nature of the 

 radial plates, Stomechinus must come amongst the Diadematidse. 

 Its position is clearly near Peclina, from which it is distinguished by 

 the imperforate and non-crenulated tubercles, together with the 

 deep branchial cuts. 



Genus HemicidaPvIs, Agass. 

 The genus Hemicidaris, Agass., is very readily distinguished from^ 



