228 PROCEEDINGS OF THE ACADEMY OF [1879. 



or ventral disk of the earlier Crinoids, and reached the conclusion 

 that the construction of the vault affords good characters for a 

 separation into families. He distinguished three principal plans 

 though he admitted the existence of a number of others upon 

 which the summit is constructed : 



1. The summit composed of a more or less pliable, sometimes 

 perhaps squamous, integument, yielding to motion in the body 

 and arms. 



2. The summit composed of solid plates with a porous ventral 

 sac, located posteriorly on the disc, and closed at the top. Anal 

 opening rarely observed, but probably lateral. 



3. The summit composed of heavy immovable plates, closely 

 joining and forming a dome arching the entire oral side. Anal 

 opening directly through the wall of the dome or at the extremi- 

 ty of a tube, the so-called proboscis. 



We have since given this subject our special attention, and find 

 that these diversities in the construction of the ventral portion of 

 the body bear a striking relation to the general arrangement of 

 the plates of the dorsal side ; that the parental genera to which 

 we have referred have each their own peculiar summit structure, 

 and that the genera into which they were subsequently subdivided 

 are characterized by mere differentiations of the original plan. 

 We find that Ichthyocrinus and its congeners, Taxocrinus, Mes- 

 pilocrinus, etc., which are embraced in almost the same generic 

 formula, possess summit structure No. 1 ; that in Cyathocriyius, 

 Poteriocrinns, Heterocrinus, and all genera with five basals, five 

 subradials, and five radials, the summit structure agrees with No. 

 2; and that the Actinocrinidae, Platycrinidse, and Rhodocrinidse, 

 with their numerous genera, are constructed like No. 3. We con- 

 sider that the last three groups are subdivisions of one great group, 

 and propose to introduce them hereafter as subfamilies. These 

 three, or, as we may say, five families thus indicated, include more 

 than one-half of all known paleozoic Crinoids. The others we 

 leave for future consideration. 



The absence of an external oral aperture is most remarkable in 

 the anatomy of the earlier Crinoids. The actinostome, or oral 

 centre, is situated beneath the vault, and forms the radial centre 

 of a peculiar skeleton of tubular passages which connect with the 

 arms. These passages beneath the vault are evidenth' homo- 

 logous with the food grooves and ambulacral canals at the peris- 



