NUCULA. 97 



long. In the Porter Collection, however, are two specimens which seem to be 

 specifically identical, and which show that its outline was sometimes more circular, 

 and in the young form more transverse and trigonal. Comparing these Devon- 

 shire specimens with Gurtonotus unio, Salter,^ which occurs in profusion at 

 West Angle Bay, Pembrokeshire, it appears that they are closely allied, and 

 possibly only varieties of the same species. My Pembrokeshire specimens 

 are unfortunately all defective, it being impossible to extricate them from the 

 hard, crystalhne, purple limestone in which they are embedded, but they appear to 

 be very similar in shape, except that they are usually more transverse and 

 quadrate than the Devonshire specimens. It is, however, clear that they 

 themselves varied very considerably in relative measurements. 



Turning next to foreign specimens, we find that Keferstein's figure of Schizodus 

 trigonus exactly agrees with our Woodwardian specimen, except that it is not quite 

 so high. This shell he identifies with Thetis (?) trigona, F. A. Eomer, which is 

 smaller, and much more transverse and trigonal, and appears exceedingly like the 

 smaller of Mr. Porter's two specimens. If this identification by Keferstein is 

 correct, there seems no reason against regarding all the above-mentioned English 

 shells as equally belonging to the same species. 



Affinities. — Schizodus ovalis, Keferstein,^ differs in being more perfectly oval ; 

 and, according to Beushausen,^ it has strong concentric striae. 



Gurtonotus elongatus, Salter,* is a very high form. Not having seen any 

 specimens of it, I am not certain whether his figures represent undistorted shells, 

 but, if so, they appear specifically distinct in outline, being long and straight 

 behind the umbo and deeply convex below. 



IV. Family — NucuLiDiE, d'Orbigny, 1843. 



1. Genus — Nucula, Lamarck^ 1801. 

 1. Ndcula lineata, PhilUiJs. Plate X, figs. 13, 14. 



1841. NucuLA LINEATA, PMlUps. Pal. Foss., p. 39, pi. xviii, figs. 64 a a — i, 



/3 a, h. 



^ 1863, Salter, ' Quart. Journ. Geo!. Soc.,' vol. xix, p. 495. 



^ 1857, Keferstein, ' Zeitsch. Deutsch. Geol. Gesell.,' vol. ix, p. 155, pi. iv, fig. 6. 



^ 1895, Beiishausen, ' Abhandl. k. Preuss. Geol. Landes., ii. s., pt. 17, p. 119, pi. x, figs. 15, 16.. 



* 1863, Salter, ' Quart. Journ. Geol. Soc.,' vol. xix, p. 496, figs. 5 a—d. 



