282 SHELLS AND SHELL-FISH. PART I. 



Unio batava of the same country,, where the teeth begin to 

 resemble those of Mysca; but they are still short, strong, 

 thick, and distinctly separated. Proceeding thus in the 

 natural series, without caring to circumscribe our groups 



by arbitrary characters, 



58 we come to P. Sicula 



(fig. 58.), a new spe- 

 cies, found by us in the 

 lakes of Leontini in 

 Sicily : here we have a 

 still nearer approach to 

 My sea than what is made 

 by Batava : the form of 



the shell, indeed, is that of Mysca ovata; but the teeth 

 are still thick, and the inner one still preserves the typi- 

 cal character of being thicker than the other. The P. 

 elongata, also from Sicily, would seem to complete the 

 union of this sub-genus with Mysca. Whether we are 

 to look to this as the union of the three aberrant genera 

 of the Unionince into one circle, or to take it as pre- 

 sumptive evidence that these shells form only a division 

 of Mysca, is uncertain. The question is, are they really 

 distinct types ? If we merely look to Mysca pictorum or 

 ovata, and Potomida litoralis or corrugata, the differ- 

 ence is very remarkable ; but on placing between them 

 Potomida Batava, Sicula, and elongata, the extreme 

 differences are softened down, and we become doubtful 

 as to the limits of the group.* There are several North 

 American shells which we suspect will clear up this point, 

 and hereafter enable us to discover the true typical cha- 

 racters. 



(264.) The HYBIN^J constitute a small but very 

 distinct sub-family, remarkable both for its characters 



* Our collection of Unionidee, upon which these remarks were founded, 

 has unfortunately passed into the hands of others, and prevents us from 

 again investigating this question. We shall feel particularly indebted to 

 those conchologists of America who study the Unionida, for specimens, ac- 

 curately named, from their " Western Waters," of all the species they can 

 part with, promising to return others in exchange, from New Zealand. 

 Letters and parcels should be directed to us, to the care of John Willis, 

 Esq., Liverpool. 



