246 
PALAEONTOLOGY 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 fi ve, 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 t ubercle 
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 arms 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 ora l 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 allocystites. 
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. IS 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.) 
