REPORT 0:S THE CRINOIDEA. 181 



help of a tegmeu, such as occurs in the Actinocrinidse. The proximal ends of these 

 tunnels would open into the closed peristomial space beneath the pyramid of apical dome 

 plates or orals. In fact, Carpocrinus appears to me to be in the condition of a Hyocviiius, 

 with an oral pyramid composed of somewhat smaller plates, but permanently closed, like 

 the ambulacra of the disk. While therefore I am in complete-accordance with Wachsmuth 

 respecting the closure of the peristome and calyx- ambulacra in the Cyathocrinidse, 

 Platycrinidge, and Actinocrinidae, I cannot altogether agree with him in denying all 

 homology between the solid vault of a Palseocrinoid and the soft disk of a recent form. 

 For I believe that both in the Cyathocrinidse and in the Platycrinidse the plates which 

 form the vault are unusually massive representatives of the ambulacral, anambulacral, 

 and interradial plates which are developed in the perisome of a Pentacrinus or 

 Comatula. 



The Ichthyocrinidae and some of the doubtful Silurian forms, such as Reteocrirms and 

 Xenocrinus, appear to me to occupy an intermediate position between the heavily vaulted 

 Platycrinidse and the more thinly plated recent forms. Some of the Mesozoic species, 

 such as Extracrinus, Ajnocrinus, Guettardicrimis, and Marsupites, which have com- 

 paratively thick plates on the sides and surface of the disk, also help to fill up the gap. 



The only genus of Ichthyocrinidas in which the summit is known at all satisfactorily 

 is Onychocrinus. Wachsmuth and Springer describe it as follows ■} — " Interradials three 

 to twenty, perhaps more in some species ; the first one large, resting between the first 

 and second radials ; the succeeding ones smaller, rapidly decreasing in size and thickness 

 upward, and having an inward curvature. They are followed by very minute, irregular 

 polygonal plates, which form the interradial portion of the vault. The radial summit 

 areas consist of two rows of somewhat larger plates, alternately arranged, which extend 

 to the ventral covering of the free rays, and probably throughout their fuU length. In 

 the median portion of the vault there are six rather thin but large apical dome plates." 

 I understand, however, from Mr. Wachsmuth that he is now less inclined to believe in 

 the presence of apical dome plates in the Ichthyocrinidse ; and I will not therefore take 

 their presence as established. If they exist I should call them oral plates, and compare 

 the vault to the disk of a Hyocrimis with a closed oral pyramid. But in their absence 

 the vault appears to me so closely to resemble the disk of Pentacrinus and Comatula, 

 that I cannot question the identity of the two for the merely a priori reason of the 

 Ichthyocrinidas being Palseocrinoids. 



The two rows of alternating plates which radiate outwards over the "squamous 

 integument," and extend on to the free rays {i.e., distichal and palmar series), are surely 

 nothing more than the covering plates of the ambulacra, which were perhaps permanently 

 closed as in the Platycrinidae, or only temporarily so, as in the Neocrinoids ; while the 

 small irregular plates which form the interradial portion of the vault, correspond to the 



• Revision, part i. pp. 53, 54. 



