276 SHELLS AND SHELL-FISH. PART I. 



unusually short; the cardinal teeth are also very peculiar^ 

 as already explained (^fig. 47. 6) ; the groove between 

 them diverges from the beak in precisely an opposite 

 direction to that which separates the lateral teeth, 

 — in other words, it is obliquely parallel to the mar- 

 gin of the shell, instead of receding from it. In the 

 comparative size of these teeth there is also a de- 

 cided difference : in lEglia, the inner cardinal tooth 

 is the smallest, while in Unio it is the largest. The 

 typical examples are JE. ovata {fig. 49.)^ cuneata Sw., 

 and occidens Lea. Unfortunately, however, the genus 

 is not so rich in forms as the last, and hence the links 

 between the sub- generic types are not so beautifully 

 graduated ; but the three species above named are 

 certainly typical, and by means of ventricosa, ater, and 

 siliqiioides, lead us to the confines of the group, where 

 we find such species as M. Say's cariosiis and ochra- 

 ceus, among which, and some kindred species, there 

 is at present much confusion : these, however, conduct 

 us to the U. radiatus of the American conchologists. 

 This is a very remarkable shell, because it seems the point 

 of junction between JEglia and the next sub-genus (J/>/*- 

 ca) ; the form of the teeth are consequently very variable: 

 hence we find that, in some varieties of this species, the 

 teeth are like those of ochraceus; in others, the cardinal 

 teeth are nearly equal, as in Mysca ; and in a very old 

 specimen now before me, the inner tooth is actually the 

 largest. These are all natural variations, and are account- 

 ed for upon the principle that osculent groups or forms 

 vary according to the characters of those other forms 

 which they connect. Lastly, we have a beautiful pro- 

 totype of Theliderma in Canthyria, the type of which is 

 the U. spinosus of Lea. This, and Naidea, are the only 

 sub-genera we shall venture to characterise ; the other 

 two requiring much m^ore study than we have yet been 

 able to give them. 



(260.) The sub-genus Mysca follows next, and ob- 

 viously blends into the last. Mysca is the representation 

 of Iridina ; and we consequently find that nearly all the 



