128 UNITED STATES GEOLOGICAL SUEVEY OF THE TERRITORIES. 



it likewise differs, however, as decidedly in general form as from the modern 

 types of that genus. 



The foregoing two, or possibly three, types are, in my opinion, very 

 nearly, if not quite, all of the known genera that can be properly referred to 

 the family Solemyidce. The very peculiar characters of the genus Solemya 

 have led the most reliable authorities on conchology to view it as the type 

 of a distinct family, standing apart from all the other existing groups of 

 Mollusca. This view was, I believe, first expressed by Dr. Gray, of the 

 British Museum. It was afterward adopted by Deshayes, H. and A. Adams, 

 Dr. P. P. Carpenter, Dr. Gill, and others. Dr. Stoliczka, of the Indian geo- 

 logical survey, also adopts the family Solemyidce ; but he includes in it 

 (provisionally only, it is true) what seems to me to be a most incongruous 

 jumble of genera, as follows: Cleidoplwrus, Pyrenomceus, Sanguinolites, Lep- 

 todomus, Orthonota, Anodontopsis, Sedgwickia, Dolabra, Grammysia, and 

 Solemya* It is certainly difficult to understand why these genera should be 

 included in the same family with such a genus as Solemya, as most of them 

 seem to have little or nothing in common with it, or with each other, 

 beyond what we see in numerous other bivalves. To say nothing of the 

 other types here mentioned, it is only necessary, in order to show how widely 

 some of these shells are removed from the family including Solemya, to state 

 that Cleidoplwrus, even in the type-species, is now known to have a crenate 

 hinge like Nucula. 



Among the more marked generic characters of Solemya may be men- 

 tioned its elongated form, very thin shell, with its anterior side so much 

 longer than the posterior, as to give it the appearance of having the ligament 

 in front of the beaks, instead of behind them ; also its shining epidermis, pro- 

 jecting beyond the free margins, together with its partly internal and partly 

 external ligament. 



The thin, light nature of the shell of Solemya enables the animal to 

 leap and swim about in the water for some time without touching bottom, 

 as was, I believe, first noticed by Dr. Stimpson. The leap is said to be per- 

 formed by means of the sudden dilation and contraction of the umbrella- 

 shaped foot ; water being at the same time forcibly ejected from the posterior 

 opening by the drawing together of the valves. 



If the type for which Professor King proposed (lie name Janeia is not 

 distinct from Solemya, this genus must have been introduced during the 



* Palasont. Indica, III, 268. 



