1890.] NATURAL SCIENCES OF PHILADELPHIA. 353 



been separated fi'oni tlie food grooves, for in the internal casts noth- 

 ing is found but the impressions of these plates, while in casts of 

 Actinocrinus, from the same locality, and in casts of certain Silurian 

 Platycrinidae, probably Cordylocriniis or Ciilicocrinus, in which the 

 covering pieces perhaps were less modified, the outlines of the am- 

 bulacra are sometimes sharply delineated, and apparently formed 

 solid tubes embracing the ambulacral vessels and food grooves. 



It is to be observe'!, that while Dr. Carpenter regarded the inter- 

 radial pieces of Platyerinus as perisomic plates, he alludes to the 

 ventral covering of that genus as a vault, including the anambula- 

 cral plates (p. 180) ; while in speaking of the ventral surface of the 

 Reteocrinidae, Ichthyocrinidae and the genus G/ypfocrinus, he 

 -generally applies the teims " disk " and " interpalmar area." We 

 allude to this fact, as he criticised us on p. 166 for using these terms 

 indiscriminately in our writings. He explains the terms as follows : 

 " The expression ' oral disk ' or ' ventral disk ' is universally used to 

 denote the upper surface of the visceral mass of a Crinoid, i. e., that 

 in which the mouth is placed with the food grooves radiating outwards 

 from the peristomial area around it." The ventral covering of 

 Platycrinus, accordingly, should be called a disk like that of Ghjpto- 

 crinxis, if it really is as, he maintains, morphologically in a similar 

 condition. 



The ventral structure of the Melocrinidae and Rhodocrinidae, 

 Carpenter probably supposed to have been in the same condition as 

 that of the Actinocrinidae and Platycrinidae, a disk when the ambu- 

 lacra are exposed, and a vault when they are concealed. He con- 

 siders Glyptocrinus in connection with the Reteocrinidae and Ichthy- 

 ocrinidae, in all of which the ventral pavement is composed of an 

 immense number of very minute, irregularly arranged pieces, which 

 in the Ichthyocrinidae are traversed by regular rows of alternating 

 pieces passing out from the mouth to the arras ; in the other fami- 

 lies, however, such alternating plates, if present at all, are found only 

 near the arm bases. Carpenter says in reference to these groups (p. 

 185), "I venture to think that in the case of Glyptocrinus, Reteocri- 

 nus and Xenocrinus, and also of the Ichthyocrinidae, the resemblance 

 to the Pentacrinidae, Apiocrinidae, and Comatulidae is such as to 

 leave no reasonable doubt that the so-called vault of these Palaeo- 

 crinoids is homologous with the ventral surface of the body in the 

 Neocrinoids." This is j^erfectly true as to Taxocrinus and Oiiycho- 

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