82 CARBONIFEROUS LAMELLIBRANCHIATA. 



frequently terminal. The shell is expanded from side to side in front, and 

 acutely bent at the anterior border to form a somewhat concave and depressed 

 anterior surface, marked off on either side by a decided curved line, which 

 arises above at the apex of the umbo. The hinge-line is straight, narrow, 

 edentulous, and shorter than the greatest transverse diameter of the shell. 



The anterior adductor muscle-scar is small, single, pit-like, and terminal ; the 

 posterior adductor scar is oval, large, situated somewhat more in the upper than 

 the lower part of the shell, and not marginal. The mantle-line is entire and 

 linear, and situated at some distance from the edge of the valve. 



Surface marked with coarse strias or large concentric folds. Shell thin. No 

 evidence of a periostracum. 



Observations. — Considered originally to belong to Inoceramus, the type of 

 this genus has been referred to Posidonomya, Bronn, by most observers, until 

 de Koninck pointed out the very distinct characters which separate it from 

 Posidonomya Becheri, the typical species of that genus. 



De Koninck, however, still retained the genus in the family Aviculidas, having 

 been unable to obtain a view of the interior and its muscle-scars. He 

 recognised only one species of the genus, but Captain Brown had described six 

 Carboniferous forms under the name " Catillus " as early as 1841 (op. supra cit.). 

 Four of these are stated to occur at High Green Wood, Vale of Todmorden. 

 Phillips describes one form as Modiola elongata, and figures another as Perna ? in 

 his ' Geology of Yorkshire.' I believe that Brown's Avicula tenera and A. squamula 

 also belonged to the same genus. Unfortunately it is impossible to trace the 

 types of many of these species, and they can be no longer retained. 



All the other forms, while possessing the characteristic general shape, 

 differ from the type Posidoniella vetusta in the surface-markings. It is from 

 specimens of these new species that I have been able to elucidate the arrange- 

 ment and position of muscle-scars and mantle-line. I have removed the genus 

 from the Aviculidas and placed it under the Mytilidas on the following grounds. 



The general shape is Mytiloid rather than Aviculoid. The shell appears to be 

 equivalve ; the posterior adductor scar is situated nearer the posterior margin 

 than in Avicula. There is an absence of ears, and the hinge-line is not elongated, 

 but shorter than the other diameters of the shell ; the pallial line is linear and not 

 dotted, and there is no evidence of. cartilage pits or grooves at the hinge. In 

 addition, the species are, as a rule, gibbose and obliquely inflated ; the umbones 

 terminal ; the hinges non-striated and edentulous. 



This genus appears to have been identified with Inoceramus on account of the 

 close resemblance between the markings of the surface of P. vetusta, the type, 

 and of many Mesozoic Inocerami. A knowledge of the hinge-characters serves 

 to separate the two genera. Woodward separated the genus from Inoceramus 



