84 AxxALs OK iiiK Cakkkcik Muskl'.m. 



ing that a constant and uniform migration of the ]:iosterior pair of 

 muscles must take j)lace. One specimen shows a sort of cardinal area 

 on the posterior margin, which is, in this individual, nearly straight. 

 There is an incomplete series of nine dorsal valves of Crania creu- 

 istriafa. They range in size from 2.66 x 3.33 mm. to 12.5 x 13.5 

 mm. It shows nothing of interest beyond the fact that there is a non- 

 plicate nepionic stage, whose shell measures 1.46 mm. in length and 

 1.66 mm. in width, on the smallest specimen. There are 25 striae on 

 this individual, all of which begin at the same time at the boundary 

 of the nepionic shell. 



Pholidops hamiltoniae Hall. (Plate I.) 



Hall, Pal. N. V., IV, 1867, p. 32, pi. 3, figs. 6-9. 



There are, in the collections, about 1,500 specimens of this s])ecies, 

 all but two of them separate valves. Next to Chonetes scitiiliis, this 

 is the most abundant species in the material. The smallest specimen 

 is .54 mm. in length and .46 mm. in width ; the largest is 3.93 mm. 

 long and 3.20 mm. in width. 



Nepionic Shell. — On the beaks of the dorsal valves of some of the 

 specimens the nepionic shells are preserved. At this stage the shell is 

 wider than long, oval, and with a hinge liiie only slightly curved. 

 The shell is conve.x and smooth. The largest shell of this stage seen 

 is .15 mm. long and .176 mm. wide. The smallest is .13 X .15 mm. 



C/iani^^es D/irini:[ Dcvclopmenf. 



After the nepionic stage, growth of the shell takes place posterior 

 as well as anterior to the beak. The shell becomes attached to some 

 foreign object by the cementation of the ventral umbo. The amount 

 of surface involved in this attachment is extremely small, so the sym- 

 metrical development of the shell is in no way prevented. On one 

 specimen the scar is roughly oval, .13 mm. long and .11 mm. wide, 

 and this is about the average size. 



The first change, in outline, from the transversely oval shell of the 

 nepionic stage is when the shell becomes elongate oval in the early 

 neanic stages. At a length of .50 to .75 mm. it assumes the form 

 which it retains throughout life. The beak is situated about one 

 quarter (.21 to .28) of the length of the shell from the posterior mar- 

 gin, and the shell is broadest at the point. 



