4 



PLATE 10. 



Page 



Fin. 1 - ^2. Orthis obi.ata. 102 



1,2,3. Individuals sliowing a gradation of sizi', fi-om tin' smallest recognized spocimciis of 

 tlie speci-'s to the half-grown forms. 



4, •'■>, 0. Individuals illii.strating a mare elongate and more gibbons form, which seems scavcely 

 sei)arablc from the others, but is still never found of larger size than fig. 0. 



7, 8, 9. 10. Individuals of larger dimensions; the last being a full-grown specimen of the 

 broad variety, one and three-fourths inches wide by one and a quarter long. 



11, 12, 13, 14. The Interior of the ventral valves of several specimens, showing some variety 

 in the vascular impressions. 



1-5, 10. The interior of the dorsal valves of the more elongate and gibbous forms. 



17. The interior of the dorsal valve of the broad variety, having the dimensions of an inch and 



a half by an inch and a quarter. 

 19, 20. Casts of the ventral valve, showing the muscular and vascular impressions. 

 '21, 22. Oasts of the dorsal valve, with impressions of the cardinal and brachial processes. 



