PTERINOPECTEN. ] 33 



largest and the second and sixth next in size ; the whole crossed by seven or 

 eight very low and indistinct undulations, and by very numerous fine concentric 

 stria3. Surface of hind wing with about twenty small, equal, alternating 

 ribs only. 



Size of an imperfect valve. Length 60 mm., height 80 mm., depth 8 mm. 



Locality. One imperfect mould from Croyde Bay is in the Museum of 

 Practical Geology. 



Remarks. The figure of this fossil is given from a wax cast taken from the 

 mould. It is very different in size, shape, and ornament from any of the 

 accompanying fossils, but owing to its own imperfection, and to the unsatisfactory 

 description of the forms with which it may be compared, there is considerable 

 difficulty in deciding its species. 



Phillips' s figure of Pecten polytriclms is similarly ornamented. It appears 

 smaller and more transverse, and its major ribs seem stronger behind than in front, 

 but it has every sign of being taken from a very much compressed and frag- 

 mentary specimen, and it is the right or opposite valve to ours. It may be 

 observed that Phillips defines two varieties, one with alternating minor ribs from 

 Brushford, and the other with equal minor ribs from Mudstone Bay. The latter 

 may prove to be a distinct species, but my belief is that the former at all events 

 is identical with ours, and that the differences visible are to be accounted for 

 either by distortion or by his specimen being only the umbonal parts of a 

 larger shell. 



Again, Avicula Ibergensis, F. A. Romer, appears from its figure to agree with 

 our fossil in shape and character, chiefly differing in being much smaller and in 

 having two major ribs on the hind. wing. Freeh, who refigures Romer's type, 

 makes it even more like Phillips's specimen than ours. It seems probable that it is 

 only a variety of the English fossil, the differences seen being simply due to age. 



Affinities. The shell identified by Romer 1 and by Freeh 3 with P. polytrichus 

 is totally different from Phillips's species, and may be easily distinguished both 

 by its ornamentation and by its very much smaller wings. 



Avicula dillensis, Freeh, 3 has a different and much simpler style of ornament 

 on the left valve ; though the right valve is more similar, differing, however, in 

 having more numerous rays and much fewer concentric threads. 



A. pectinoides, Sowerby, 4 approaches it in size and shape, but has many more 

 major ribs, and its ornament is, as is shown by M'Coy, 5 of an entirely different 

 nature. 



1 1860, F. A. Eomer, Beitr. Harzgeb.,' pt. 4, p. 161, pi. xxv, fig. 5. 



2 1891, Freeh, ' Abhandl. Geol. Specialk. Preuss.,' Band ix, pt. 3, p. 16, pi. i, figs. 9 9 b. 



3 Ibid, p. 39, fig. 2, pi. iii, fig. 9, and pi. xiv, fig. 17. 



4 1840, Sowerby, ' Q-eol. Trans.,' ser. 2, vol. v, pt. 3, pi. liv, fig. 2. 



5 1855, M'Coy, 'Brit. Pal. Foss.,' p. 393. 



