172 Dr. G. J. ll'mdeon a 



above the other (PI. VI. fig. 14). These spines are really 

 portions of the thin flanges of the larger rings of the stem, 

 and they indicate the position of the larger rings, which have 

 been covered over by the cirrus-bases. 



The points of difference between this form and H. itlia- 

 censis, as described by Prof. Williams, clearly indicate that 

 it is a distinct species. The calyx is nearly twice the size, 

 and whilst the tubercles on the corresponding plates are nearly 

 thrice as numerous, the spines themselves, particularly those 

 of the vault plates, are not more than one third the length of 

 those of H. itliacensis. The interbrachial plates are also 

 more numerous, and there are no indications of smooth rows 

 of plates in the vault, as is stated to be the case in this latter 

 species. Hitherto no cirri have been discovered in connexion 

 with the stem of H. itliacensis, but it is probable such will 

 be found to be present in this species as well. 



Prof. Williams compares //. ithacensis with specimens of 

 an undescribed form in the Museum of Cornell University, 

 of which, he states, Prof. Hall has privately distributed the 

 photograph with a name attached. As no description of these 

 specimens has ever been published, and as I have not the 

 opportunity, like Prof. Williams, of examining them, I cannot 

 institute any comparison between them and H. Carpenteri. 



An interesting feature in connexion with this species is the 

 fact that out of eleven examples which have been discovered 

 there are three in which a gastropod shell of the genus Platy- 

 ceras, Conrad, is attached to the vault of the crinoid, which 

 it nearly entirely covers. Though the shells have been some- 

 what displaced by the compression of the crinoids, there is 

 clear evidence that the sinuosities of their margins, when in 

 their original positions, very closely fitted to the surface of the 

 crinoid vault. It is also noticeable that, in all three instances, 

 these shells have a similar very definite position in relation 

 to the anal aperture of the crinoid, so far as it can be deter- 

 mined by the situation of the anal plate ; and they are so 

 placed that the front margin of the gastropod shell would just 

 project over this aperture. There can be no doubt that the 

 gastropod derived its support from the materials rejected by 

 the crinoid, whilst, at the same time, the position it had taken 

 up does not appear to have in any way injured the crinoid ; 

 at all events, the specimens to which the shells are attached 

 are larger and more perfect than the others. 



The Platyceras-shelh belong to two species : one, with the 

 surface smooth, merely showing sinuous lines of growth, is 

 closely related to Platyceras erectum, Hall* ; whilst the other 

 * See Palaeont. New York, vol. v. p. 5, pi. ii. figs. 4, 11. 



