OF SOUTHEKN INDIA. 393 



the shell ; hinge with several transverse sub-external ligamental pits, anterior and 

 posterior to the umbones with a few long ribs, generally placed obliquely towards 

 the hinge line, and besides often with numerous shorter folds at its margin ; type, 

 G. soleiwides, Defr. Species of this genus (as restricted) are known from all the 

 mesozoic deposits ; they are most numerous probably in the middle Jurassic period ; 

 only very few species have been recorded from tertiary deposits. 



With regard to the genus Bakewelia of King, Avhich is by some authors con- 

 sidered to be closely allied to Avicida, I have already (p. 335) observed that its 

 classification is somewhat doubtful ; it either belongs to the family Arcid^ where 

 I described it, or it has to be placed in the present family close to Gervillea, 

 between this genus and Hdvues'm. 



19. Iiiocemmus, Sow., 1819, (Parkinson in Trans. Geol. Soc, Lond., 

 V, p. 59, and Sowerby, Min. Conch., vol. iii, pi. 305 ). Roundly oval, or more or less 

 elongated, and often higher than long, equivalve or sub-equivalve, inequilateral, 

 beaks sub-anterior, sometimes with a rudimentary ear in front, below which there 

 is a shallow insinuation ; hinge line straight, posteriorly more or less produced, 

 thickened at the margin, and provided with numerous ligamental pits ; hinge inter- 

 nally edentulous ; extei'nal layer of the shell generally very thick, consisting of 

 vertical fibres, internal layer thin, pearlaceous ; type, J. Cuvieyiaiius, Sow., from cre- 

 taceous beds. The genus occurs from the silurian up to the close of the cretaceous 

 period, gradually increasing in number and variety, but does not appear to pass into 

 the tertiary period. The older forms are often found with the outer fibrous layer 

 not preserved, while the pearly layer remains ; this is also often the case with 

 species occurring in gault-beds, but those in the upper cretaceous have, on the con- 

 trary, the fibrous layer preserved, but not the pearly one. This diff'erent mode of 

 preservation is most likely due to the aragonitic or calcitic character of the two 

 shell layers, as pointed out by Sorby and others. The name Inoceramus has been 

 by some palneontologists reserved for the mytiloid-shaped species, while Catillus was 

 applied to the more inflated and elongated species, like I. Lamarckkmus, but the 

 one form passes so insensibly into the other, that no generic or sub-generic separa- 

 tion of the two can be retained. Both are characterised by concentric sulcations 

 and more or less regular striae of growth, sometimes crossing the former. These 

 strise belong, like in lacva or Gyclas, merely to the superficial layer of the shell, 

 which generally consists of a peculiar glassy substance. 



The literatiu'e of the numerous cretaceous species is partially very confused, 

 and the discrimination of species unsatisfactory. It is to be hoped that the 

 desire expressed by many of possessing a monograph of the genus will soon be 

 satisfied. There are few other fossils more characteristic for the different sti'ata 

 of the cretaceous period than the Inocerami. 



19rt. Act'mocemmns (Check-list cret., N. A. fossils, Smith. Misc. Coll., 

 No. 177, 1864, p. 32,) has been proposed by Meek for a small group of Inocerami 

 of the type I. sulcatus. Park. They have a rather short and somewhat oblique 

 hinge line, the left valve is often slightly more globose than the right, and both are 



