368 rROCEEDINGS OF THE ACADEMY OF [1890. 



they were exposed only to near the upper ends of the ridges, and 

 then followed the inner floor. If this interpi-etatiou is correct, 

 GlyptocrinuH represents another instance in whitth a sort of vault is 

 formed by the lateral extension of tlie interanibulacral system of 

 plates. 



An examination of the different tegmens which occur in the 

 various families of the Camerata, leads to the conclusion that the 

 ambulacra, as a rule, are subtegminal in specimens with a high 

 dome and bulging at the arm bases ; but are generally tegminal, or 

 become tegminal before entering the arms, in species with a flat or 

 depressed surface. They are also exposed in species with high 

 ridges, and in those in which the arm openings are directed upwards, 

 a structure which necessitated a rising of the ambulacra toward the 

 arms. The presence or absence of the ambulacra upon the tegmen, 

 therefore, is not, in our opinion, an essential structural feature in it- 

 self, but is rather a consequence of differences in the form and con- 

 struction of the tegmen in the respective species. This explains in 

 a natural way why the ambulacra are exposed in forms like Adino- 

 crinus quinquangularw, Habrocriinis ornatus, Marsupiocriitns de- 

 pressus and Glyptaster ornatus, all of which have a more or less 

 flattened ventral surface, and conspicuous elevations reaching up to 

 the orals, and why in the typical Actinocrinus with a high conical 

 dome the ambulacra are subtegminal, but tegminal in the depressed 

 Actinocrinus stellaris which approaches the condition of the Platy- 

 crinidae. In Platycrlnus the tegmen is rather flat in ail elongate 

 species, and all have more or less high ridges. The latter we 

 find also in the short discoid species with an elevated dome, owing 

 probably to the shortness of the dorsal cup. The condition of the 

 ambulacra in Platycrinus may also be due partly to the extrava- 

 gant development of the plates. 



Comparing the earlier Melocrinidae and Actinocrinidae with those 

 from the Devonian and Subcarboniferous, it is seen that they are 

 built substantially on the same plan. Among the latter the ambu- 

 lacra occasionally form a part of the solid test ; while in by far the 

 majority of their species they are kept below. The plates in all of 

 them occupy relatively the same position to surrounding parts, the 

 only perceivable difference is that those of the later forms are some- 

 what larger and heavier, which is readily explained by palaeouto- 

 logical development. AVhy then should their tegmen be composed 

 of a system of plates morphologically distinct from that of the others? 



