412 CRETACEOUS PELECYPODA 



met with in palaeozoic beds, though also rarely. In the Trias, the number 

 increases considerably, rising to about 30 or 40 ; the species belonging mostly to 

 Madula (as restricted) and to Flagiostoma, Erom the Jurassic period there must 

 be about 120 species known : JPlagiostoma is very largely represented ; next comes 

 Radula^ then Limatula and the peculiar Ctenostreon, Erom cretaceous rocks 

 there are at present about 204 species on record, and the number is still rapidly 

 increasing with the examination of the various horizons of this period, in which 

 the family evidently attained its greatest development. All the sub-generic divi- 

 sions are found there represented ; and very probably when our materials have 

 been improved, it will be found convenient to arrange the costate forms, now 

 generally referred to Radula, into several sub-genera. Erom tertiary beds there 

 are barely more than 60 species known, and in the present time, this number 

 has been reduced nearly by one-half. Of Limea there are only four species known, 

 but it is not improbable that a few of those described as Badulce will be shown to 

 belong to Limea, I will note some of those species. 



In comparing the cretaceous species known from the so-called Old Continent 

 with those of the New Continent, it is noteworthy to observe that the present 

 geographical distribution of the recent forms was already indicated at that early 

 date, or perhaps it was even more marked. Out of the 204 species of cretaceous 

 Madula and its sub-genera, there are only twelve which have been found in 

 America. 



1. Madula, Klein, 1753, (Ostrac, p. 135, pi. ix, fig. 34, Lima, Brug., 1792). 

 Shell obliquely sub-ovate, generally moderately inflated, equivalve, radiately ribbed 

 or striated; beaks prominent, pointed, more or less distant from each other; hinge- 

 line with unequal ears on either side, generally somewhat sloping from the beaks, 

 with a small triangular area between them and a sub-median cartilage pit in it, 

 extending somewhat internally ; muscular scar sub-centric, pallial line entire, both 

 faint; type, Madula lima, Linn., a recent species which already occurs in the miocene 

 strata of the Vienna basin. I can see no essential difference between the recent 

 Mediterranean forms, those from the Eed Sea and those from the Mcobar Islands, 

 where I collected the species in great numbers. 



The recent species are few and rather general in their distribution, though none 

 of them are very common shells. Several useful sub-genera have been distinguished 

 by H. and A. Adams in their " Genera of recent Mollusca." I have only little to 

 add to these. 



\a. The name Madula must be restricted for the type species M, lima, Linne, 

 which IS probably the same which Klein quotes as MumpUana, and which occurs 

 at the Nicobars. The shell is rather strongly radiately ribbed, oblique, moderately 

 compressed, the anterior or concave side having a comparatively narrow gape. 

 The ribs are either smooth, or concentrically striated, or squamose. The ere- 

 taceous period contains a large number of species belonging to this division, 

 but some of them are rather of a peculiarly rounded shape, with inflated valves, 

 approaching the next sub-genus. 



