CLASSIFICATION OF THE BRACHIOPODA. 109 



connate with their bases ; the muscular impressions left by the adductor are large, pro- 

 duced, and elongated, occupying more than two-thirds of the length of the valve, and 

 bordered by well-defined ridges. In the interior of the dental valve the muscular scars are 

 small, and not distinctly margined ; the adductor lies close to a slight mesial ridge, while 

 the cardinal muscles leave larger scars on either side ; vascular impressions radiating. 



Ohs. On comparing with attention the shells above characterized with those forms we 

 have admitted into Stropliomena, such as B. alternata and depressa, the observer is struck 

 by the great difference observable in the shape and position of the muscular impressions, 

 especially in the interior of the socket valve : this character is constant, and essentially the 

 same in Lept. Iransversalis, sericea, ohlonga, Bavidsoni, tenuicincta, quinquecostata, &c. ; 

 other differences likewise exist in the development of the socket ridges and cardinal process. 

 It would, perhaps, therefore be advisable and convenient to retain for these species the 

 term Leptcena, especially as one of Dalman's types is comprised in this section, and by so 

 doing, the general opinion in favour of preserving the two names Strophomena and Leptana 

 is deferred to ; in this view I am supported by Mr. Salter, who had long ago come to the 

 same conclusion. Professor King admits the generic claim of this group, but would 

 prefer the name Plectambonites, as he feels desirous of retaining that of Leptana for such 

 shells as L. depressa, and L. analoga, &c. ; but we have already expressed under Stropho- 

 mena, the objections to that arrangement. Some of the species in this group, (Z. Bavidsoni, 

 L. ohlonya, &c.) are likewise perforated by a small circular foramen, for the passage of a tem- 

 porary pedicle, proving how unfounded were M. d'Orbigny^s views when characterising 

 Strophomena with a perforation, and Leptcena without one ; but as the interior of a vast 

 number of species of so called Strophomena and Leptmna are still unexamined, it would be 

 premature to come to a ^cW(/ conclusion on the subject. 



Geol. range. — Some of the shells composing this genus appeared for the first time in 

 the oldest Silurian period, and continued with very similar shapes upwards and through 

 the Lias, above which no examples have been hitherto observed. 



Examples ; L. iransversalis, Dalman ; sericea, J. Sow. ; ohlonga, Pand. ; Bavidsoni} 

 Ed. Deslongchamps ; quinquecostata, M'Coy ; tenuicincta, M'Coy ; transversa, Pand. ; 

 convexa, Pand., &c. ; liasiana, Bouch. ; Bouchardii, Dav., &c. 



Sub- Family ?— DAVIDSONID^. 



Animal unknown ; shell fixed to marine bottoms (rocks, shells, and corals) by a portion 

 of the dental or ventral valve ; hinge line straight, with a more or less developed false area, 



1 The discovery of this remarkable species in the upper Lias of Normandy is due to Mr. Perier, of 

 Caen, and has been described and illustrated in M. Eugene Deslongchamp's valuable paper on the 

 Leptsense and Thecideae of the Lias. L, Bavidsoni measures 10 lines in width, and bears the most 

 striking resemblance in external shape to some Russian Silurian specimens of L. transversa. 



