6 Trans. Acad. Sci. of St. Louis. 



which is never wanting ; and secondly a laterally expanded 

 blade, not present in the young but becoming gradually 

 more and more dev^eloped as they grow older; see Plate 

 VI. The first or base portion, in the typical species 

 above mentioned, has a shape resembling very much a six- 

 sided prism; see Plate II., Fig. 9. The surface toward the 

 center of the calyx has a semi-lunar groove which forms 

 with the adjoining piece the central opening or mouth, and, 

 as this basal part is placed in a slightly oblique position, it 

 causes the external opening to be a little smaller than the 

 internal one. The opposite side of this semi-lunar cavity 

 is prolonged into a narrow blade, except for the posterior one, 

 where the septum is divided into two blades for the outlet 

 of the anus. The lower part is divided lengthwise into two 

 blades, running down and outwards with a plicated outer 

 margin, — Plate II., Fig. 9, — for the support of the plications 

 of the hydrospiric tubes, whereas the upper part of this 

 blade forms a sharp crest, and the outer one a triangular in- 

 cision, during the juvenile state, for the reception of the 

 upper points of the fork pieces which rest in this triangular 

 incision. At the base of these lamellar blades we find a semi- 

 lunar groove which forms with a similar one on the base 

 of the lancet piece the genital opening, except in species 

 where the deltoid pieces are perforated, as in G. norwoodi 

 and similar forms. The outer or upper surface of the base 

 portion is either rounded or sharp pointed, whereas the lower 

 surface, which expands toward the center of the calyx, is 

 grooved transversely so as to form a circular groove around 

 the mouth on the interior side of the calyx for the reception 

 of the central ring of a water vascular system. This central 

 water ring underlies the basal part of the deltoids and does 

 not penetrate the calcareous shell, sending off five straight 

 branches, one for each ambulacrum, running between the hy- 

 drospiric tubes and the lancet piece, but not through the 

 latter; see Plate II., Fig. 8. 



The outer half of the prism is transversely perforated by a 

 fine channel, forming in connection with the other parts a 

 pentagonal ring for the reception of the nervous center. 



