10 CARBONIFEROUS LAMELLIBRAXCHIATA. 



Observations. — Phillips (' Geol. Yorks.,' pt. ii, 183(), p. 211) described three 

 species of shells under the generic name GercilUa — G. lavdnosa, U. hitmlatu, and 

 O. squamosa ; and they possess a very marked external resemblance to this genus, 

 but have not the series of perpendicular fossettes for the ligament in the cardinal 

 area. On the other hand, they have a well-marked prolongation of the hinge-line 

 to form a rostrum. A fourth species was referred to this genus, G. incousplcno, 

 which was imperfect, and the type has unfortunately disa23peared, so that it is 

 impossible now to hazard a guess as to the identity of this specimen, but it has 

 seemed to me that it might belong to Sauguinolitcs striatolamellosus. 



De Koninck (' Descr. Anini. Foss. Belg.,' 1843, p. 129) and others referred 

 Phillips's three species to Avlcula, with which genus they have even a greater 

 resemblance ; l)ut the hinge-plate is markedly diiferent, for there seems to be no 

 long lateral tooth, but a striated hinge-plate. Bal-oireUkt, King, has a closely allied 

 form, but has, according to that author, several cartilage pits in the hinge area, 

 like Penia and Gervillia. 



PteriiuYi, Goldfuss, also has certain characters in connnon, but the hinge-plate 

 with two diverging lateral teeth and the thick shell are not present in LeI.opteria. 



Hall described tAvo genera, Lclojdevia and Lrpfoch-snia, which are very closely 

 allied, if not the same genus. According to this author, " Lcptodesma, in its 

 prevailing forms, is similar to Leiopteria, except that the anterior end is always 

 nasute and acute, instead of auriculate and rounded." In shape the British 

 Carboniferous shells are certainly more nearly like the figured species of Leptodesnia 

 than Leiopteria, and have the elongate process posteriorly common to Arint/a and 

 the former genus. It seems doubtful to me if both generic names are necessary. 

 It is interesting to note the very small differences between Leiopteria and recent 

 forms of Avicula. Externally and in general shape the genera resemble each other 

 very closely ; the general plan of the hinge is the same, l)ut the posterior lateral 

 tooth is stronger, shorter, and more oblique in Ariaiht, and the cardinal teeth seem 

 to be smaller and placed further forwards in Ljeiopteria. Indeed, had I not 

 consulted one of the most eminent living conchologists on the point, I should have 

 considered the differences of hinge character as of specific value only. 



De Koninck has described twenty-one species of Leiopteria from the Carbo- 

 niferous rocks of Belgium, of which eighteen are said to be new. A study of the 

 figured specimens will show that many of these are young or immature forms of 

 other species. 



