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Show 1895.] MR. F. A. BATHER ON UINTACRINUS. 995 bifascial articulation ; but there is no defined ridge, and of course no such skewing from right to left as in the brachials. The pinnules gradually decrease in size towards the distal end of the arm, and eventually become of exceeding tenuity. Since they are rarely preserved or traceable for their whole length, measurements are difficult to make. A brachial 3*0 m m . wide bears a pinnule of which the proximal ossicle is 1*0 m m . wide. A brachial 2*5 m m . wide bears a pinnule of which the proximal ossicle is *8 m m . wide, and the total length more than 11*5 m m . A brachial 2*0 m m . wide bears a pinnule of which the proximal ossicle is about *5 m m . wide, and the total length fully 16*5 m m . 3. THE EELATIONS OF UINTACRINUS. As already stated, there are no forms with which direct comparison is obvious. The discovery of the ancestry of Uintacrinus must therefore be a long process of induction. In prosecuting such an inquiry, the first step is to clear away secondary and accidental characters, so as, in any comparison, to utilize only those that are essential. It is, for instance, futile to lay any stress on the fact that Uintacrinus is an unstalked crinoid, and for that reason to compare it with its contemporary Marsupites. Worse still to follow H . A. Nicholson and P. H . Carpenter (7), and to place the two genera in a single family, though their organization differs in almost every other respect. As well place it with Saccocoma, or with Agassizocrinus, or any other stalkless crinoid. Surely the argument is absolutely the reverse. Features in which unstalked and free-swimming crinoids agree with one another are, it is probable, features due to similarity of environment rather than similarity of descent. The resemblance is physiological, not morphological. In short, one infers that such features are secondary, and not essential. They are the ones to be cleared away. Let us consider the general and common characters of unstalked crinoids. All agree in the absence of a stem in the adult; but, when further compared, they are soon seen to fall into three distinct groups. Pirst, the group in which a portion of the stem remains, becoming modified into a cirrus-bearing centrodorsal, as in Antedon, Eudiocrinus, and Thaumatocrinus. These forms anchor themselves by their cirri, and though capable of crawling, climbing, and swimming, do not often exercise their faculty of locomotion. Secondly, the group in which either a portion of remaining stem, or the lower part of the cup (i. e. basals or infrabasals), becomes solidified, usually by additional deposition of stereom, into a knob, which, one may suppose, serves as ballast or as a sea-anchor; such forms are Agassizocrinus, Edriocrinus, and Millericrinus pratti. Both of these groups have a small calycal cavity with thick walls, and there can be little doubt but that all are attached by a stem in the earlier stages of ontogeny. The third group, comprising Marsupites, Saccocoma, and Uintacrinus, has no trace of a stem or |
