RHYNCHONELLA. 85 



Obs. Thirteen specimens of this small species have been obtained in the Inferior 

 Oolite of Somersetshire, by the officers of the Geological Survey, and now deposited in 

 their Museum; as remarked by Professor Forbes, all the examples are so similar and 

 constant in shape, that it is not very probable that they are young individuals, as we do not 

 find any other similarly plaited globular Rhynchonella in our Oolites, to which we could 

 refer it; it somewhat resembles R. Pisum, but can be distinguished without difficulty 

 from the Cretaceous species, which is a flatter and more delicately plaited shell. 



Fig. 19. Natural size, from a specimen in the collection of the Geological Survey. 

 „ 19 a4c . Enlarged illustrations. 



79. Rhynchonella serrata, Sow. Sp. Plate XV, figs. 1, 2. 



Terebratula serrata, Sow. M. C, vol. v, 1825, p. 168, tab. 503, fig. 2. 



— — Morris. Catalogue, 1843. 



— — Tennant, 1847. A Strat. List of British Fossils, p. 74. 



— — Bronn. Index Palseont., 1848, p. 1250. 

 Rhynchonella serrata, D'Orb. 1849. Prodrome, vol. i, p. 289. 



Diagnosis. Shell irregularly subtrigonal, generally a little wider than long, valve 

 almost equally convex, but depressed; beak small, much recurved, and lying over the 

 umbo, leaving little space for the passage of the pedicle muscular fibres; beak ridges well 

 defined, exposing a slightly concave false area between them and the hinge margin, a 

 similar depression being likewise visible on either side of the umbo, the hinge line not 

 much indenting that of the smaller valve. Surface ornamented by a variable number of 

 large sharp plaits, proceeding from the beak and umbo to the front and sides, from 

 fourteen to fifteen on either valve, generally no distinct mesial fold or sinus ; six or seven 

 plaits are sometimes a little raised in front in the smaller valve; structure imperforated. 

 Length 15, width 17, depth 10 lines. 



Obs. This beautiful species was well described and figured by Sowerby from a small 

 specimen stated to have been found in the Lias of Lyme-Regis, whence, however, I have 

 never obtained an example, and I believe it probable that Sowerby's type was found 

 in the Marlstone of Ilminster, whence many fine specimens have been obtained by 

 Mr. Moore. Sowerby alludes to the shell in the 'Ency. Meth.,' (PI. 243, fig. 11, and 

 PI. 244, fig. 1,) as being possibly referable to the species under consideration. We do not, 

 however, believe them identical, although allied; the shell of the 'Ency.' is Terebratula 

 plicata of Lamarck, which is a Rhynchonella} and seems distinguished from R. serrata by 

 being less depressed, deeper, longer than wide, and ornamented by fewer costae, seven or 

 eight on each valve ; some specimens presenting four plaits, others five on the mesial fold, 

 and in a few, two uniting into one towards the margin of the shell; characters different from 



1 Lamarck, 'An. sans Vert.,' vol. vi, 1819, No. 39; and Dav. on Lamarck's species, ' Annals and Mag. 

 of Nat. Hist.', vol. ii, 2d ser., June, 1850, PI. xiv, fig. 39. 



