48 CARB0N1C0LA, ANTHRACOMYA, AND NAIADITES. 



attachment of the posterior end of the external ligament, as is the case in the 

 recent Anodonta. 



The umbones of this species are seldom eroded. The only specimens in which 

 I have seen this feature are from Codnor Park. One specimen from the Collection 

 of Mr. Neilson, of Glasgow (see woodcut), appears to have been bored with a 

 circular hole; and a similar hole is shown in one of Mr. Salter's figures (op. supra 

 cit.). To what these holes are due it is impossible to guess, for Gasteropoda 

 have never been found in the beds with Garbonicola (see also p. 74). 



The pit for the reception of the cardinal tooth of the opposite valve is very 

 large in the specimen figured, PI. II, fig. 1 a, much larger than is necessary. The 

 same feature may be observed in many of the valves on Pis. V and VI, and more 

 particularly in the transverse section on PI. XI, fig. 2. This raises a question 

 as to the possibility of there having been present an internal ligament or cartilage 

 lodged in this hollow, and the fact that striae can be made out would also point to 

 this conclusion ; but this cartilage could not have been always present, as there 

 is no room for it in many of the hinges figured on Pis. V and VI differing from 

 that which I have considered to be the normal type. It is possible that this 

 hollow lodged a process of the external ligament, which had here its anterior and 

 strongest attachment. Whilst, however, on the one hand there are forms showiiig 

 a slighter degree of development in the posterior end, there are others (PI. II, 

 figs. 1,2), from Limerigg, Slamanuan, in the collection of Mr. Smith, of Kilwin- 

 ning, which show a more elongated shape, with much smaller comparative dorso- 

 ventral diameter ; but these have the characteristic constriction at the junction 

 of the middle and posterior thirds of the valves, and the beaked posterior end. 



This species is the largest form of Garbonicola, and is very much larger than 

 other members of the genus. In England the large form has a very limited 

 distribution, both horizontally and locally. It is much more common in Scotland ; 

 though, there, it appears to keep to one horizon. In the North Midlands of 

 England there is a dwarf form, but generally obtained crushed. 



The shape is very characteristic ; the comparatively great dorso-ventral depth 

 and the tumidity in the anterior half of the shell are of diagnostic importance. 

 Salter considered the form to be identical with TJnio rugosux (Pachyodon, Brown). 



