468 SUPPLEMENT. 



are extremely elongated and plain, the striated portion (or palpi) almost obso- 

 lete, whereas ia Placuna the plicated surface is sufficiently extensive. The 

 outer gill-laminre, in both genera, are furnished with a broad reflected margin. 



flicatida, p. 259. The animal is like Spondylus in every essential re- 

 spect, and only resembles Ostrea in the foot being nearly obsolete. 



Streblo-pteria (laevigata) McCoy, 1856. Carb. Brit. {Aviculida). 



Mytilidce. Modiola pelagica {Myrina, Adams), p. 266, has the mantle 

 open ; the shell is peculiar from the large size of the anterior muscular im- 

 pression ; and the subcentral umbones distinguish it from Modiolarca. 



Eoplomytilus (crassus) Sdbgr. Devonian, Nassau. Shell with a mus- 

 eular plate in the umbo, like Septifer (p. 265). The 3fylilus squamostis, 

 Sby. Magnesian limestone, Brit, has a similar plate. 



Arcades. Scaphula (celox) Benson, the fresh-water Ark, p. 268. A 

 second species has been found in the R. Tenasserim, Birmah. The hinge is 

 edentulous in the centre, and the posterior teeth are laminar and branched ; 

 the elements of the posterior muscular impression are distinct. 



Limopsis, p. 268. Sy7i. Pectunculina, D'Orb. Mr. M'Andrew has dredged 

 L. pygmcea, living, on the coast of Rumark ; it is a fossil of the Pliocene of 

 England, Belgium and Sicily. 



iY«CM/;^cs,p.270. 

 The Yoldia limatu- 

 1-^ /« has been dredged, 

 <— (pnrr rrrrrr.N\^ :^<:::;^ y^ ""^ ^^X^ i alive, by Mr. M'An- 

 drew, on the coast 

 of Finmark. It is 

 also found in Port 

 landHarbour,Mass. I 

 The animal is very active, and leaps to an astonishing height, exceeding in 

 this faculty the scollop-shells. (Dr. Mighels.) 



Unio7iidcB, p. 276, Miilleria; Fig. 246 represents the left, or attached 

 valve, showing the single muscular impression, and projecting spur with the 

 nucleus, consisting of both valves of the fry, united, and filled up with shell.* 



HippurilidcB, p. 279. The structure of these shells has been more fully 

 described in the Quarterly Journal of the Geol. Soc. London. In all the 

 genera the shell consists of three layers, but the outermost, which is thin 

 and compact, is often destroyed by the weathering of the specimens. The 

 principal layer in the lower valve of the Hippitrite is not really veiy difierent 

 from the upper valve in structure ; the laminae are corrugated, leaving irre- 

 gular pores, or tubes, parallel with the long axis of the shell, and often visible 

 on the rim. The umbo of the upper-valve of the Radiolite is marginal in 

 the young shell. (Geol. Journ. vol. xi. p. 40.) 



* M. D'Orbigny very liberally placed his suite of specimens of this remarkable 

 genus in the British Museum. Oct. 18M. 



/ 



^...t- )j 



Fig. 263. ToWio Zimaiw/o (after Barrett). i.„.itt_..i Tir-._ i 



