214 Messrs. E. Etheridge, Jim., and P. H. Carpenter on 



The basis of the classification whicli we have been led to 

 adopt is tlie morphology of the hydrospires and of their ex- 

 ternal openings, the so-called spiracles. We find that the 

 structure and distribution of these organs, together with the 

 arrangement of the various elements comjiosing the amb.ulacra, 

 present characters of much systematic value. 



Besides discovering various new generic types, both British 

 and foreign, we have been led to form entirely new concep- 

 tions of some of the already existing genera, e. g. Oroplto- 

 crimis [Codonitcs] and Granatocrinus ] while we are able to 

 give more precise definitions of Troosiocrinus and of Tricoclo- 

 crinus than have hitherto been attemptetl. 



In order to facilitate the comprehension of our specific 

 descriptions, Ave give the following explanation of our termi- 

 nology : — 



The plates forming the calyx are the basals, radials or fork- 

 pieces, and the deltoid pieces or orals *. 



In Codaster trilohatus and in the more flat-topped American 

 species of the genus the middle line of each oral is occupied 

 by a more or less strongly marked ridge, the oral ridge. But 

 in other species of Codaster and in the allied genus Plueno- 

 schisma this ridge is represented merely by an edge, from 

 which the sides of the orals slope sharply downwards towards 

 the ambulacra. This oral ridge is often very prominent at 

 the central ends of the oral plates of Pcntremites^ so as more 

 or less completely to separate the proximal ends of the passages 

 which lead to the hydrospires of adjacent ambulacra. 



In most Blastoids each radial is more or less fork-shaped, 

 the hcindle of the fork being the body of the radial (which is 

 separated from the basals by the hasiradial suture), while the 

 two prongs are the limhs. Ijctween tliem is the radial sinus, 

 which is occupied by the ambulacrum. This terminates in a 

 more or less prominent " /?}; ^' on the upper edge of the body 

 of the radial f . 



Of the ambulacral structures which together fill up the 



* As lar as we can jutlg-e from various passages on pp.12 and 13 of 

 tlie ' Eevision of tlie Palseocrinoidea ' by Messrs. Wachsmiitli and 

 Springer, tliese anthors seem to hold the same view as -vve do respecting- 

 the homology of the deltoid plates of the Blastoids -vvitli the " consoli- 

 dating phites " of Cyathocrinits and the orals of the Peutacrinoid. But 

 they make no definite statement to this effect. 



t According to Messrs. Wachsmnth and Springer, "The lower part of 

 the forked p] "" +o the sr^hulacriim is the first radial — in Blastoido- 

 irimis, the oh. .v'nlJlastoid, the ,<utnre is visible — and the two sides 



of the fork, i bemg interradial, form together a second radial "' 



(Revision, ■ \\'e are sorry that we cannot agjee with the American 



pali;eontolog..-.is in this view. "NVe have nnfortnuately been unable to 



