216 ]\ressrs. E. Etlierldge, Juu., and P. H. Carpenter on 



tliese processes. The hydro.spire-]iores, therefore, are formed 

 without the intervention of any " ])ore-plates," which, for this 

 and otlier reasons, we prefer to call " side plates." 



Tlie hydrospire-canals open externally bj the spiracles^ 

 which are arranged at the summit, round the peristome ; they 

 may be either single {Granatocrimis) or paired {ScMzohlastus^ 

 Troostocrinus) . The spiracle or spiracles of the anal inter- 

 radius may be confluent with the anal opening to form the 

 anal spiracle. 



In the better-preserved examples of many American Blas- 

 toids the mouth and ])eristome are concealed beneath a vault 

 of minute irregular summit-plates^ any definite arrangement of 

 which is rarely traceable. This vault is sometimes continued 

 down tiie ambulacra by a series of tiny covering -plates ^ which 

 close in the food-grooves completely and convert them into 

 tunnels just as in recent Crinoids. In none of the Euro[)ean 

 Blastoids that we liave seen has any indication of these struc- 

 tures been preserved. 



As some considerable time must still elapse before the pub- 

 lication of our memoir, we wish to direct the attention of our 

 fellow workers to one or two morphological points of interest. 



In the ordinary Pentreniites the hydrospiral tubes beneath 

 the ambulacra extend alone; the entire length of the radial 

 sinuses and communicate with the exterior by the marginal 

 pores ; but in the little P.conoideus, Hall, and P. Koninclcianus^ 

 Hall, of the WarsaAV limestone *, the hydrospiral tubes ap{)ear 

 to be absent from the terminal third of the ambulacra. 'J'lie 

 sinus is considerably shorter on the inner than on the outer 

 aspect of the radial ; and the hydrospiral lubes end abru])tly on 

 the inner face of the plate, while the ambulacra extend much 

 further towards the base of the calyx. Exaraj)les of both 

 species are common with the shell filled up by a foraminiferal 

 matrix, which thus forms a complete cast of its interior. This 

 may be readily exposed by the removal of the shell ; and it is 

 then seen that tlie length of the ambulacra externally is 

 greater than that of their internal casts. We have been un- 

 able to detect this peculiarity in any other species of Pentre- 

 mites, but we think it very probable that our American col- 

 leagues may be more fortunate. Sections through the u]:)per 

 part of the calyx of P. conoideus show that it possesses hydro- 

 spires constructed on the ordinary Pentreniites ty])e. We have 

 }iot, however, been able to obtain thoroughly satisfactory sec- 

 tions through tlie distal portions of the ambulacra ; but from 



* We caiuicl help suspectiiij^' the identity of tlie^e twu .so-called 

 species. 



