NIAGARA GROUP. 
245 
tinated apertures on the right, and the supra-ovarian plate, there is a small plate 
intercalated in both specimens, as shown in the figures; and in the upper figure, 
there are two small plates above the higher pectinated one. 
Fig. 7. A diagram of the structure of the upper part of a specimen where the arms are pre¬ 
served, showing their arrangement in two pairs as described. The filaments of os- 
sicula are shown, extending along the groove to the little points which represent the 
bases of the tentacula. 
Fig. 8. A part of the preceding diagram enlarged, showing the plates at the base of the arms, 
the position of the mouth, and anal pore in reference to the other parts. The ovarian 
pyramid is in the centre below : on the left side is the oral aperture a, and below it 
the minute anal pore d, b b representing the bases of the tentacula. 
Fig. 9. The brachial plates on the left side enlarged, showing the oral and anal orifices, with 
the bases of the two nearest tentacula. 
Fig. 10. A part of one of the arms enlarged, showing the filament of ossicula running along the 
groove, and the bases of the tentacula b b. 
Fig. 11. A still farther enlargement, showing the first joints of the tentacula. 
Fig. 12. A lateral view of the same. 
Fig. 13. An enlargement of the ovarian pyramid and the surrounding plates. 
Fig. 14. Profile of the same, showing the elevation of the pyramid. 
Fig. 15. One of the pentagonal ovarian plates, with the three adjoining ones at the base, much 
enlarged. 
Fig. 16. The pectinated plates of one of the higher pairs, showing the form and proportions of 
the two apertures. 
Fig. 17. Another plate enlarged, to show the tuberculous character of the surface. 
Position and locality. This species was found with the preceding at Loclcport, and has not 
been observed elsewhere. {Collection of Col. Jewett.)■ 
Genus HEMICYSTITES (nov. gen.). 
[Gr. r\ fju, semi , and xuoVk, vesica.] 
Body circular, depressed at the margins, centre elevated, composed of an unequal number 
of imbricating plates; arms five, adhering, radiating from the centre, and composed of a 
double series of alternating joints ; an ovarian orifice closed by triangular plates ; an oral and 
an anal orifice, with a porous tubercle near the apex. 
This body was at first supposed to be an immature stage of some crinoid ; but after examining- 
several specimens of this very singular fossil, I have felt compelled to constitute a new genus 
for its reception. It is parasitic upon shells, and being composed of a large number of unequal 
plates, with the peculiar arrangement of the arms and the other more vital organs, it seems 
sufficiently unlike any known form to be regarded as distinct. It has, in its present condition, all 
the characteristic organs of a cystidean, wanting the form and arrangement of plates; and 
therefore if we regard these bodies as higher in the scale than crinoids, the fossil under con¬ 
sideration, if immature, must be regarded as exhibiting the embryonic condition of a still 
higher organization. 
