NIAGARA GROUP. 



245 



tinated apertures on the right, and the supra-ovarian plate, there is a small plate 

 intercalated in hoth 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, h 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 i h. 



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 Lockport, and has not 

 been observed elsewhere. {Collection of Col. Jewett.) 



Genus HEMICYSTITES(7ioj;. gen.). 

 [Gr. V'l semi, and xutfTi?, vesica.'] 



Body circular, depressed at the margins, centre elevated, composed of an unequal number 

 of imbricating plates ; arms live, 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 crinold ; 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. 



