52 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



Fig. 33. Fig. 34. 



Fig. 33. Cast of ventral valve of M. barrisi. 

 Fig. 34. Cast of ventral valve of M. haskinsi.* 



In the dorsal valve of M. barrisi we have a hinge-plate, with a median 

 septum reaching more than one-third the length of the shell ; and the same 

 characters exist in M. haskinsi. In 31. doris, the rostral cavity and mu- 

 scular impression of the ventral valve are much elongated, and resemble 

 what I have heretofore shown in Meristella Icevis.^ The dorsal valve has a 

 strong extended median septum, and hinge structure as in the other species. 



The proportions of length of rostral cavity and muscular impression 

 vary in different species ; and the muscular impression becomes much 

 stronger and deeper in the older shells, when the valve, as before remarked, 

 becomes thickened at the sides and towards the beak. This character per- 

 tains to the limestone specimens; while those in the Hamilton shales, as 

 figs. 7 & 8, have thinner shells, and less deep and strong muscular impres- 

 sions. 



I have already ( Thirteenth Report on the State Cabinet, pp. 73 - 75, 

 and illustrations on p. 93 ) pointed out the distinction between Athyris 

 = Spirigera and Meristella. This difference is everywhere clear and 

 unmistakable, in the external lamellose surface of the one, and the almost 

 smooth character of the other. The muscular impressions of the ventral 

 valve of Athyris are at once distinguishable from those of Meristella, 

 as may be seen on comparison of figs. 35 & 36 with figs. 29 & 30. 



Fig. 35. 



Fig. 36. 



* The casts of M . barrisi and M. haskinsi are obtained from solid specimens by 

 removing the shells, and therefore have not that sharpness of the muscular markings 

 which we find in weathered casts. • 



t Palaeontology of New-York, Vol. ill, plate 39. 



