BIVALVIA. 63 



tripartite division of the exterior is of unequal dimensions, the posterior striae covering 

 nearly but not quite half the surface, while the anterior occupies rather a less space than 

 the centre or naked compartment ; the striae or rays are large and rounded, number- 

 ing about a dozen or fourteen on the anterior side, with about double that number on 

 the posterior portion : the whole shell is covered with transverse striae or regular 

 lines of increase, which prettily ornament the spaces between the ridges, and 

 the edge of the shell is deeply crenulated on the anterior and posterior sides, or 

 those portions which are covered with the radiating ridges ; the tripartite division 

 of the shell, is visible in the interior, and the number of the external stria? may be 

 counted there. The edge of the ventral margin has a slight convexity, contracting a 

 little towards the striated parts. 



The differences between this species and the following are so evident, there 

 can be no mistake, that shell being more compressed or less tumid, with the posterior 

 side broader in proportion. The shell to which this approaches nearest, is 

 M. semi-nuda, Desk., 'Desc. des Coq. Foss. des Env. de Par.,' vol. i, p. 264, pi. 30, 

 figs. 20 — 22, a fossil belonging to the Formations of the Older Tertiaries. I have not 

 been able to obtain a specimen from the Paris Basin ; but what I presume to be the 

 same species in the Cabinet of Mr. Edwards, from the English Deposits, presents 

 differences that may be regarded as specific. Mr. Edwards's shell is more regularly 

 ovate, and is even thinner than our species, with fewer radiations on the anterior 

 side, not having more than seven or eight, and these are broader, it is also, more 

 regularly tumid than our own shell, which has somewhat of an obtuse angle on the 

 posterior portion. 



The shell figured and described by Dr. Gould, under the name of M. discors, seems 

 to present but trifling differences with the British shell, judging from description alone ; 

 but it is considered to be distinct by British Conchologists, as well as by Dr. Loven. 



7. Modiola discors, Zinnaus. Tab. VIII, fig. 5. 



Mytilus discors. Linn. Syst. Nat., ed. 12, p. 1159, No. 261, 1767. 



— discrepans. Mont. Test. Brit., p. 169, 1803. 

 Modiola discrepans. Turt. Brit. Biv., p. 202, 1822. 



— — Desk. 2d ed. Lam., t. vii, p. 23, 1835. 



— — Forbes. Malac. Monens., p. 44, 1838. 



— — Mailer. Ind. Moll. Groenl., p. 19, 1842. 



— — Alder. Cat. Moll. North, and Durh., p. 81, 1848. 

 Modiolaiua discors. Loven. Ind Moll. Scand., p. 33, 1846. 



Crenella discors. Forb. and Hani. Hist. Brit. Moll., vol. ii, p. 195, pi. 45, figs. 5, 6, 



and pi. 48, fig. 5, 1849. 



Spec. Char. Testa ovato-ettipticd, subcompressd, valde inaquilaterd, tenui ; antice et 

 postice striata, spatio mediano lavigato ; latere postico latiore. 



Shell ovato-elliptical, somewhat compressed, very inequilateral, thin ; striated at 



