106 Mr. H. Woodward on the Tertiary Shells 



thickening of the shell renders their contour very dissimilar 

 to the normal type. Such varieties as these, if met with in 

 older deposits, would without doubt be treated as of specific 

 value. 



Anisothyris [Pachydon] ovata, Conrad, op. cit. 

 pi. 10. fig. 4. 



I look upon this species as probably founded on the young 

 state of A. Hauxvcelli^ with which it very well agrees, save 

 that it is said to be loMte : this, however, may be the result of 

 bleaching. 



Anisothyris carinata^ Conrad, op. cit. pi. 10. fig. 7. 

 PI. V. fig. 6. 



" Shell triangular, very inequivalve ; right valve pro- 

 foundly ventricose, but flattened on the disk ; posterior slope 

 flattened, having an indistinct ridge in the middle, and form- 

 ing nearly a right angle with the umbonal slope, which is 

 slightly carinated ; posterior extremity acutely angular ; left 

 valve prominently angular on the umbonal slope, concave 

 anterior to it, and depressed on the posterior slope, with a fine 

 raised line in the middle of the slope ; posterior ventral margin 

 nearly rectilinear. This shell is covered with a very thin, 

 pale, shining epidermis, and varies greatly from the typical 

 species." 



I have nearly a hundred specimens of this species, which 

 appears to be most characteristic. The only form at all 

 approaching it is the P. ohliquus of Gabb, which, in some of 

 its extreme varieties, assumes the angular form of carinata. 



Anisothyris {Pachydon) ohliqiia^ Gabb, op. cit. ; Conrad, 

 op. cit. pi. 10. fig. 15. PI. V. figs. 5 a, h. 



We possess a remarkably fine series of this species, num- 

 bering 830 specimens. 



Shell very inequivalve ; the valves extremely oblique, the 

 posterior angle being drawn out, and the anterior compressed ; 

 so that the longest axis of the valves is from the umbones to 

 the ventral margin, instead of from the posterior to the ante- 

 rior side. The umbones are highly spiral, some examples 

 ending in a perfectly free convolution ; on the other hand, we 

 have been able, from this large series, to pick out examples in 

 which the spiral umbones are altogether suppressed. 



In form this species closely resembles the recent genus 

 Cardilia ; but this shell is costated, whereas the valves of the 

 fossil form are smooth. Cardilia^ moreover, has a broad plate 

 inside on the dorsal margin of the left valve ; and the umbones 



