OCR Text |
Show 986 MR. F. A. BATHER ON UINTACRINUS. [Dec. 17, so low as IIBrT. The succeeding interdistichal usually stretches across the interdistichal area, but it may be split longitudinally in two. Also it may be succeeded by more interdistichals, from 1 to 5. The number of interdistichals is not necessarily constant in the interdistichal areas of one individual. Considering this extensive variation, it is a little odd that it should not have been noticed by any of the American writers. Similar variation is not known in the unique specimen of U. westfalicus, which in this portion of its anatomy agrees with the normal U. socialis. The interpinnnlars are small plates that are often intercalated in the spaces between tbe proximal portions of the fixed pinnules and the intervening IlBr. There is usually only one interpinnular between two adjacent pinnules, but sometimes there are two. A n interpinnular on one side of an area has usually a corresponding interpinnular on the other side. The interpiunulars are quite common : I have seen them in about half of the specimens under examination, e. g. y, e, el, f, g, k, I (figs. 4, 5, 8, 10, 11), and it is a little curious that they are neither figured nor mentioned by any previous writers. W e have seen that the fixed pinnules probably arose from free pinnules becoming involved in the construction of the cup. We may inquire now as to the origin of the interbrachials and interdistichals. Are they derived in whole or part from pinnules, or from the tegmen ; or are they mere supplementary plates ? The variation in number might show that they are degenerate pinnular elements; but it is equally consonant wdth the theory of their supplemental origin. There is the very slightest trace of any linear arrangement, and in such rare cases as might plausibly be imagined to represent a degenerate pinuule, such an assumption would upset the regular sequence of pinnulation ; so that one cannot adopt this view without further proof. I can distinguish no difference in this respect between young and old individuals. The constancy of the proximal interbrachial might lead one to regard it as a primitive tegminal element, since an interradially situate plate of the tegmen is often conspicuous in just such a position. But the proximal interdistichal, which can scarcely be a primitive tegminal element, is nearly as constant. The presence of interpiunulars, which are undoubtedly intercalated, supplementary plates, show how all these plates may have arisen. Considered as supplementary plates, the interbrachials may primitively belong to either the cup or the tegmen; one can regard all interbrachial plates as portions of the tegmen that have become included in the dorsal cup. The general relations of all the component elements and areas of the dorsal cup to one another are as follows :-The fixed primibrachs and secundibrachs form slightly raised ridges, more prominent distalwards. The interbrachial areas, including interbrachials, fixed pinnules, and interpinnnlars, are slightly depressed at the sides, but form a slightly elevated shield-like area. The interdistichal areas are depressed. These points are more obvious |