38 Mr. W. King on certain Genera 



answering to the deltidial valve*. There are one or two more 

 differential characters which ought not to be overlooked in draw- 

 ing up a diagnosis of these genera : for example, in Leptcena the 

 deltidium is open and wide at the base, but in Strophomena it is 

 usually narrow and cicatrized ; and the hinge plates are often cre- 

 nulated in the latter but generally smooth in the former f. As 

 regards internal characters, they appear to be more inconstant in 

 Strophomena than in Leptcena : StrophomenaDutertrii has the mus- 

 cular f ulcra of the ventral valve elevated, curving over the mesial 

 plate and united, by which means they form an arch-shaped pro- 

 cess J ; while in S. deltoidea they are reduced to mere impressions : 

 in S. lepis the fulcra of both valves are slightly elevated, and re- 

 semble those of Chonetes : certain species, as S. transversalis t S. 

 Humboldti and S. oblonga, have lateral (? ovarian) spaces, one or 

 more on each side of the mesial line, bounded by a prominent 

 ridge, as occasionally seen, though less obviously, in Chonetes ; but 

 in most of the genus there is no appearance of a bounding ridge : 

 in general the vascular system has not left any impressions on the 

 inner surface of the valves, — the exception, as far as I have been 

 able to ascertain, occurs only in Strophomena lepis, which in this 

 respect resembles the Leptsenas generally. The very converse of 

 this mutability of internal structure prevails in Leptcena, the spe- 

 cies of which are remarkable for the similarity of their muscular 

 fulcra, for generally exhibiting impressions of the vascular system, 

 and for the general absence of ovarian (?) bounding ridges. Some 

 of the Leptsenas arrest our attention by their being foraminated ; 

 that is, besides possessing an open deltidium, they are furnished 

 with a circular opening in their slightly prominent beak. I pos- 

 sess several specimens of a species, apparently the same as Lep- 

 tcena analoga, from the carboniferous shales of Northumberland, 

 exhibiting the foramen in a very obvious manner ; whether it is 

 general to the genus I cannot say. My Swedish specimens of 

 Leptcena rugosa do not appear to possess this character, but it is 

 seen in an allied species from the Eifel. Strophomena alternata 

 possesses a similar opening, which is the only instance in this 

 genus that has been made known § ; but it does not appear to be 



* I know of only one exception, which obtains in Strophomena euglypha : 

 in this species the deltidial valve is the concave one. The like exception 

 probably occurs in Orthis, since 0. crenistria and some other species have 

 the same valve affecting the concave form. 



•\ Leptcena nodulosa, Phillips, has crenulatcd hinge plates. 



% This at least is the view I formed from a hasty examination of this sin- 

 gular apophysis in a specimen belonging to the beautiful collection of Mr. 

 W. A. Lewis of Wolverhampton. I have specimens of Productus Martini 

 and Leptcena analoga (?), in which the muscular fulcra are unusually ele- 

 vated and curved over the mesial plate, but they do not coalesce as in Stro- 

 phomena Outer trii. 



§ Geology of Russia, vol. ii. p. 225, &c. 



