122 Mr. H. Seeley on Cambridge Palceontology : — 



marked by excentric folds. Muscular impression very elon- 

 gated. 



Not uncommon. Cambridge and Ashwell. Coll. University. 

 J. Carter, Esq. ; Mr. Farren. 



Mr. Carter possesses a cast of the exterior of Perna lanceolata 

 showing the shell to have been lamellar^ like that of an oyster. 

 The Pernce appear to have commonly produced pearls in prodi- 

 gious numbers. These are often met with detached, sometimes 

 as large as peas : they mostly have a yellowish colour. The 

 easts of shells are often marked in the lower part with radiations 

 like the Unionidse. But, besides the free pearls, pearls attached 

 to the shell were far from uncommon, and in some individuals 

 so numerous as to remind one more of artificial productions in 

 the Chinese Anodon than of a natural growth. No nacreous 

 specimens have been found which can satisfactorily be considered 

 as attached pearls; but their impi'essions, left on phosphatic 

 casts, are unmistakeable. They mostly occur on the upper part 

 of the shell. The specimen figured (PI. VI. fig. 6), collected by 

 Mr. Blake (Caius Coll.), had sixty- five such pearls, and is a fine 

 example of their occurrence. 



Pholadomrja decussata, var. triangulata. 

 Form triangular, subequilateral, somewhat rounded at the pos- 

 terior extremity, inflated. The anterior and posterior outlines 

 are nearly straight. The anterior side is abrupt, being bent 

 ' at about right angles with the shell, and has descending from 

 the umbo, and near to the hinge, an inflation, between which 

 ^ and the keel formed by the bend is a slight depression. The 

 inflation of the shell from the anterior side to the posterior 

 extremity is very much less than that from the umbones to 

 the base. The umbones prominently recurved. Posterior side 

 * steadily rounded. The whole shell is ornamented with excen- 

 ' trie folds with slight depressions between them : in the young 

 state they are very narrow ribs. These are crossed by fine 

 radiating ribs placed close together ; they are not present on 

 the truncated portion ; and though, in the young state, ex- 

 tending over the remaining part, those of the posterior third 

 become obsolete, with a diameter of about half an inch. One 

 rib, however, near to the hinge-line descends to the base. 

 This fossil appears to be a dwarfed variety of P. decussata 

 (Mant. sp.). All the specimens are about an inch in each dia- 

 meter. 



Rare. Coll. University ; J. Carter, Esq. ; G. D. Liveing, Esq. 



Several free valves have been found of an attached bivalve^ 



