124: AxNAi.s OF THE Carxe(;ie Museum. 



ventral valve. There is a convex chilidium, at the apex of which is 

 the minute anal opening. The inner opening of the anal tube is at 

 the anterior base of the cardinal process, just in front of the point 

 where it bifurcates. The cardinal process in the young stages is wider 

 than long, projects little beyond the hinge line, and is divided once. 

 In the adult it is longer than wide, deeply bifurcated in front and 

 quadrifed on the posterior face. On each side of it is a dental socket 

 which is bounded posteriorly by a slightly developed lamella. 



Muscle Scars. — In the dorsal valve there are two pairs of small 

 adductor scars, separated by a thin median septum which extends half 

 Avay to the front of the valve. 



In the ventral valve there are two small adductor scars and in front 

 28 29 of them two larger diductor impressions. The 



scars are not strongly marked in either valve. 



Rhipidomella vanuxemi Hall. 



Fig. 28. Rhipido- 

 mella vanuxemi Hall : I'^"' ^'^l- N- V., 1867, pp. 40, 47, pl- 5> %• 6, pi. 6, fig. 3. 



A ventral valve .show- There are about 150 shells belonging to this 

 ing the comparatively gpecies in the collection. Nearly all the small 



larije number of strioe at u 11 j r tu 1 4. • i 4.u 



■^ shells and many of the larger ones retain both 



a verv early stage, and .... , . r^i ^ 



, ' , , , valves in their proper relations. Ihe large ones 



the smallness or thane- - ' ° 



pionic shell. V 16. ^^^ not well preserved and the largest one which 



Yw,. 29. The same retains its form with sufficient completeness to be 



species. Outlineshow- measured, is only 15 mm. in length, while there 



ing relative convexity ^^.^ fragments of larger ones, some indicating a 



of valves of a shell .58 , ,, ^ ^ ■ ^ ■ rr^i 1 



, , T, ,. shell of more than twice that size. ihe larger 



mm. in length. 1 edi- ° 



eel valve above. X16. shells are, however, rare. The smallest shell is 

 .42 mm. in length and .61 mm. wide. 



The largest shell of this species, figured by Hall, is 27 ram. long 

 and 32 mm. wide. The smallest, 13.5 x 15 imii. 



Xepionic Stage. — The non-plicate shell in this species is extremely 

 small, varying from .10 to .25 mm. in length. Both valves are 

 convex, without fold or sinus. The outline is subcircular, the length 

 and width nearly equal, and the hinge arcuate (Figs. 28, 29). 



Changes During Developmeut. 

 Outline. — In the early neanic stages the shell is wider than long 

 and the frontal and posterior margins are nearly parallel, which gives 

 the shell a pronounced transverse appearance. The index is from 



