234 



PALEONTOLOGY OF NEW-YORK. 



In addition to these peculiar and characteristic organs, there are, in several genera at least, 

 others, which have sometimes been termed " amhulacral openings" but which Prof. Forbes 

 terms " pectinated r/iombs." These organs are arranged in pairs (each pair forming a rhomb| I]) 

 on contiguous plates, and have a general similarity to each other, but they are by no means 

 precisely alike. They consist of a more or less elevated border or rim, which is quite thick 

 and strong, evidently intended for the protection of the enclosed part, and the centre is de- 

 pressed below the surface of the plate. In the bottom, these spaces are marked transversely 

 (which is usually in a line nearly vertical to the body) by slender bars, with interspaces equal 

 to the bars, from which they arc termed pectinated. It is certain that these organs offered 

 a means of communication between the interior and the exterior, perhaps not unlike that 

 afforded by the pores of Caryocrinus and Hejiicosmites ; but the dissimilarity of the inili- 

 viduals of each pair, on adjoining plates, and their distribution in three parts of the body 

 only, indicates that their office was not so universal, and probably more specific than that of 

 the pores. Nevertheless these apertures open upon the interior precisely as upon the exterior ; 

 and from the base of each, to the margin of the plate between these pairs of organs, there 

 extend little grooves. In the other direction, the grooves are marked for a short distance at 

 least ; but the specimens examined do not furnish positive evidence that they extended to the 

 margin of the plate*. 



In their external form, these pectinated apertures, as just remarked, are similar, but not 



" dorsal pole of the cuticular skeleton. Diametrically opposite to the orifice for the pedicle is placed the buccal orilice, 

 " and generally close to it is the subcentral anal orifice. The cup differs, however, from that of the criiioids, by 

 " such a predominance of the dorsal side over the ventral, that the latter is often reduced to a minimum, consisting 

 " only of the orifice of the mouth, so that the arms appear to be much nearer to the mouth than is the case with 

 " crinoids" (Volborth on the Arms of Cystidese : Trans. Min. Soc. of St. Petersburgh, IS-JS-B, p. 1B7). 



There is certainly reason to regard this argument in reference to the structure of these bodies, and the origin of 

 the arms, as compared with the true Crinoides. In the Caryocrinus, we observe in a remarkable degree the de- 

 velopment of the dorsal side, with a diminution of the ventral, particularly in young specimens, where the summit 

 is proportionally narrower than in older ones. This body is provided with true arms and tentacula, while Hemicos- 

 MITK5, which is so nearly related in structure, amhulacral pores, etc., has the summit protruded, and a lateral or 

 posteal ovarian aperture. The examples of Caryocrinus where the mouth is forced downwards by the absence of 

 the scapular plates, or the modification of the costal plates leaves that organ much in the position, and having the 

 appearance of an ovarian aperture. In the absence of arms, therefore, the Caryocrinus would become a true 

 Cystidoan, with the exception of the mouth. Among the analogies of Caryocri.nus to the Cystideans, may be 

 noticed the tentacula, which are composed of a double series of joints, alternating with each other precisely like 

 the arms of Cystideans, and unlike the tentacula of any other crinoid which has fallen under my observation. It is 

 not probable that Caryocrinus is the only form where intermediate characters will be found; and we shall pro- 

 bably find a necessity of restraining subdivisions within narrower limits. 



' Prof. Forbes will pardon me for differing from his opinion in reference to these organs being media of communi- 

 cation with the interior. I am quite sure he could not withhold his assent to this opinion, were he to examine the 

 specimens I have before jne. Moreover I can not conceive it possible that such conspicuous organs, and so peculiarly 

 arranged with reference to the animal, and the exsert parts to the protection of the interior, should be only modifica- 

 tions of a plate, or a metamorphosis of any part, except to serve some very important function in the animal economy. 



It is not impossible but these narrow spaces may become obsolete by coalescing of the bars, as do the pores in 

 Caryocrinus; or that in the process of mineralization of the specimen, the spaces may have been filled with 

 crystalline matter, taking the same character as the adjoiuing, originally solid, parts. 



