THE HYDH08PIBES \M> sen; LCLES. 87 



communicate with ;i water-vessel situated in the middle line of the ambulacrum; 



while on the other hand it had also been pointed out that the marginal pores of 

 Pentremites lead down into hydrospire-sacs at the sides of the ambulacrum, and thai 

 pores are absent both in Codaater and in Orophocrinus ; so that these types could 

 have had no tentacles and must therefore have differed very considerably from the 

 remaining Blastoids. There is also a difficulty in understanding how a "soft and 

 membranaceous " tentacle could have become transformed when collapsed into a 

 solid limestone plate, the form of which varied according to the species of Pentre- 

 mites. This difficulty was pointed out by one of us ' ; but Hambach's only reply 2 

 was to reiterate his previous statement, and to assert that the tentacles " form in 

 their collapsed state the supplementary poral plates of Roemer, which, to the great 

 surprise of Mr. Carpenter, are actually found preserved in an open condition from 

 the Carboniferous period to the present time." He likewise gave a " transverse 

 section of a restored ambulacral field," in which each hydrospire-sac is represented 

 as communicating with a marginal tentacle, with what appears to be a sucking disc 

 at its end, just as in an Urchin. If, as asserted by Hambach, the outer side plates of 

 a Blastoid are the remains of collapsed tentacles, why are no outer side plates ever 

 found in any fossil Urchin which, when living, must have had tentacles in abun- 

 dance 1 and, secondly, where are the pores through which the tentacles were pro- 

 truded in Phamoschisma nobile, P. Arehiavi, and P. YerneuiU, all species which have 

 well-developed outer side plates'? (PL XI. fig. 4; PL XIV. figs. 6, 9). 



Hambach's statements, although put forward as matters of absolute fact about 

 which there can be no possible doubt, are in reality nothing but expressions of indi- 

 vidual opinion, which it is difficult to take seriously on account of his ignorance of 

 some of the most elementary facts of Echinoderm morphology. 



B. Descriptive. 

 1. The Hydrospires. 

 Although it is probable that the presence of hydrospires beneath or between the 

 ambulacra is one of the essential characters of a Blastoid, it must not be forgotten 

 that there are two genera in which these organs have not as yet been detected, viz. 

 Pentephyllum and Stephanocrimis. The first-named is only represented by an imper- 

 fect cast of the calyx (PL XVI. figs. 14-16), while the nature of the ambulacral 

 structures of Stephanocrinus is still very obscure. There is no external indication of 

 hydrospires in Astrocrinus, though traces of them appear upon the inner faces of the 

 radial plates (PL XX. figs. 4, 18, 20), just as in Cryptoschisma, Orophocrinus, and 

 Triccelocrinus (PL XVI. figs. 3, 4, 6 ; PL XVIII. fig. 11), but the spiracles of this 



1 Ann. & Mag. Nat. Hist. 1881, vol. viii. p. 4z'3. 



2 Trans. St. Louis Acad. Sci. 1884, vol. iv. no. '3, p. 5U9. 



