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PAL-EONTOLOGY OF NEW-YORK. 



608. 1. HEMICYSTITES PARASITICA. 



Pl. LI. Figs. 18, 19, 20 and 20+. 



Body more or less circular, composed of plates of unequal sizes, the outer ones overlapping 

 the edges of the inner, central plates large, a border of smaller ones surrounding them and 

 forming the margin ; arms five, radiating from the centre and reaching more than half way to 

 the margin, appressed and coalescing with the plates of the body, composed of a double series 

 of plates which interlock with each other at their joining edges ; ovarian aperture closed by 

 a double series of plates, five outer ones, and an equal number within these ; a porous tubercle 

 near the apex on the side opposite the ovarian orifice, and near this the minute oral orifice, 

 with an obscure pore below and near it ; surface of plates granulated ; base closely adhering 

 to the shell on which it grows. 



This peculiar body is parasitic on the shells of Spirifer niagarensis and S. radiatus, ad- 

 hering so closely that it is scarcely possible to separate them. In the more perfect specimens, 

 the characters mentioned in the description are readily recognized. The plates never join by 

 their edges directly, but overlap and lie against each other. The plates within the radius of the 

 arms are larger, and sometimes nearly equal in size, those of the margin beyond the radius of 

 the arms are smaller, gradually decreasing to the margin, and form a close slightly thickened 

 border. The space between the arms, enclosing the ovarian aperture, is wider than between 

 any two other arms of the body. The arras appear to consist of two pairs extending right and 

 left, and of one anterior one : those of each pair meet together before reaching the centre, and 

 are united by a stronger ridge of plates, from which the anterior one proceeds. There is not 

 sufficient evidence to decide that there were tentacula attached to these arms, except perhaps 

 near the summit. The outer limit of the ovarian aperture is on a line with the radius of the 

 arms, so that its apex is a little nearer the outer margin than the centre. The oral orifice is so 

 minute and obscure that little can be said of it, more than that near the porous tubercle, on 

 the side opposite the conspicuous ovarian pyramid, there is a triangular orifice, and near it a 

 circular depression or pore. The existence of this porous tubercle is very curious and interesting, 

 since it has been shown to occur on the very distinct globose body of C alloc ystites. 



Fig. 18. An individual of the natural size. 



Fig. 19. The plates of the ovarian pyramid enlarged, showing a double series, one a little within 



and alternating with the other. 

 Fig. 20. The specimen No. 18 enlarged, showing more distinctly the arrangement of the plates, 



arms, etc. 

 Fig. 20 + . A still farther enlargement of a part of a specimen, showing the arrangement of the 



arms, the position of the ovarian aperture, the oral and anal orifices, with the 



comparatively large porous tubercle near the apex, on the side opposite the ovarian 



aperture. 



Position and locality. In the shale of the Niagara group at Lockport. 



{Collection of Col. Jewett.) 



