ISOCHILINA. 15 



no means cover the whole ostracodal fauna of the beds. Time did not permit 

 their full study before they were put on the plate, and consequently they were then 

 arranged according to their apparent general similarity. A more minute examina- 

 tion proved that in some cases these resemblances were deceptive, and were due 

 simply to the crushing or corrosion of specimens having really different characters. 

 While this has caused some of the figures to be useless as far as the elucidation of 

 their own species is concerned, it is perhaps not uninteresting as showing how 

 easily one genus may apparently be transformed into another by a slight modifi- 

 cation of its characters. 



I. Family — LEPERDiTiiDiE, Jones, 1868. 



1. Genus — Isochilina, Jones, 1858 (and 1870). 



1. Isochilina canaliculata, Krause. PI. II, figs. 13 — 15 ; and PI. Ill, figs. 1 — 2 b. 



1892. IsocHiLiXA CANALICULATA, Krause. Zeitsch. Deutsch. Geol. Gesell., 



vol. xliv, p. 385, pi. xxi, figs. 

 1 a, b. 



Description. — Valve suboblong, generally rather short, moderately convex, 

 with a central spot or muscle-mark. Dorsal margin rather shorter than the 

 valve. Free margins bounded by a definite rim. 



Size. — Length 2 mm. 



Localities. — In the Porter Collection is a slab with several specimens, from 

 Poles Hill ; and in my collection two specimens from Upcott Arch Quarry, and 

 one from Saunton Hotel. 



Remarks. — These specimens all appear to be casts ; they seem always to show 

 the central spot or dent. 



Professor Rupert Jones has identified the more perfect specimens with 

 Krause's species. He considers they come very near to Primitia valida^ but 

 are distinguished by the ventrally protrudiug convexity of the valves, and by 

 the outstanding lips or borders. 



From P. centralis, Ulrich,^ they seem to differ chiefly by being more convex. 



Primitia cestrensis, var. caldwallensis, Ulrich,^ and P. suhsequata, Ulrich,* are 

 also so similar as hardly to be distinguishable except in the same particulars. 



1 1886, Jones and Holl, ' Ann. Mag. N. H.,' ser. 5, vol. xvii, p. 409, pi. xiv, figs. 7 — 9. 

 - 1890, Ulricb, ' Journ. Cincin. Soc. N. H.,' vol. xiii, p. 130, pi. x, figs. 1—3 ; and 1893, Jones, 

 ' Quart. Journ. Geol. Soc.,' vol. xlix, p. 291, pi. xii, fig. 1. 



* 1891, Ulrich, ' Journ. Cincin. Soc. Nat. Hist.,' vol. xiii, p. 201, pi. xiv, figs. 7 a — c. 

 •i Ibid., p. 202, pi. xiv, figs. 8 a—c. 



