136 Annals of thf, Carnecwe Museum. 



as described by Beecher and Schuchert. (Proceedings Biol. Soe. 

 Wash., Vol. VIII, p. 73, pi. X, fig. i.) This is the second genus of 

 brachiopods in which this stage of the loop has been observed and its 

 presence serves to confirm the view expressed in the paper referred to, 

 that the Centronella form of loop is a primitive loop for this super- 

 family of brachiopods. It differs slightly from the loop of Dialasma 

 turgida {Loc. cit., pi. X, fig. i) in that the lamelUe are narrower and 

 the angle in front is not so acuminate. 



Cyrtina hamiltonensis Hall. (Plate V, Row 6.) 



Hall, Pal. N. Y., 1867, p. 268, pi. 27, figs. 1-4, pi. 44, figs. 26-33, 38-52. 



Of this species there are about one hundred specimens in the col- 

 lection, most of them immature individuals. The smallest shell is 

 .53 mm. long and the width is the same. The largest is 9.13 mm. 

 long and 13.33 '''^'''"'- wide. 



Nepionic Sliell. — The smallest individual in the collection repre- 

 sents the species in the nepionic stage. The shell is nearly circular in 

 38 39 40 41 outline and the length and 



breadth are the same, . 53 mm. 



The hinge line is short, .32 



mm., and nearly straight. 



Fig. 38. Cyrtina hannllonensis Hall ; out- ^^j^^ ^tXiix^X valve has a high 



line of the smallest specimen. V 28. t 1 1 • 1 ■ 1 



^ , , . ' ,-1 X - ^ cardinal area which is almost 



1*10. 39. A larger specimen ; profile. >: lb. 



Fig. 40. The same; outline. X 16. entirely occupied by a large 



Fig. 41. A slightly older individual, show- pedicle opening. The dorsal 



ing the rapid change in the relative convexity beak is erect and the space 



of the valves. X 16. below it is also occupied by 



the pedicle oj)ening. Both valves are convex, the ventral only a trifle 

 more so than the dorsal. No trace of the fold or sinus appears at this 

 stage. 



Changes During Dcvclopuient. 



In Outline. — Specimens less than i mm. in length are usually but 

 little wider than long. The index varies in neanic stages from 1.33 

 to 1.50 and in the adult stages from 1.45 to 1.75. The width at the 

 hinge is, in all stages, usually a little less than the greatest width below, 

 sometimes equals, but very rarely exceeds it. 



Convexity of Valves. — Specimens in the nepionic and earliest 

 neanic stages, with a shell .75 mm. or less in length have the valves 



