BIVALVIA. 105 



and prominent denticles, divided into two portions; connexus ligamental, placed in a 

 triangular fossette ; two adductor-muscles ; impression of mantle without a sinus. 



The connector being situated entirely on the outside of the dental apparatus, concen- 

 trated in a triangular cavity and opening the valves by contraction, is a sufficient character 

 to entitle these shells to be placed in a distinct generic position. This triangular fossette 

 bears a resemblance to that upon Limqpsis, but there is no extended area or bipartite 

 division of the connector, as in that genus. 



M. Nyst proposed the name Trigonoccelia for those bivalves which resembled Pectun- 

 cuius, but differed in the disposition of the connector, as before remarked. Trigonoccelia, 

 therefore, from want of priority, had lapsed into a synonym. M. Deshayes has employed 

 the above name for his genus, as the type species had been called Trigonoccelia by M. Nyst. 



The present genus is an emanation from Limopsis, differing from it by the loss of the 

 expanded ligamental area, approaching closely to Leda in the form of the shell. The 

 animal is at present unknown, but from its pointed siphonal region it probably possessed 

 incipient siphons. 



A few species only of this genus have been described ; six of these are from the Paris 

 Basin, and one has been figured by Mr. Lea from the Eocene deposits of America ; these, 

 with the British species, are all confined to the older Tertiaries. The animals appear to 

 have been capable of firmly closing their valves ; they have large and well-marked impres- 

 sions of the adductors. 



1. Trigonocoslia deltoidea? Lamarck. Tab. XIX, fig. 11, a — c. 



Nucula deltoidea? Lamk. Ann. du Mus., t. vi, p. 126, and t. ix, pi. 18, fig. 5. 



— — J. Sowerby. Min. Conch., t. 554, fig. 1 . 



Limopsis deltoidea ? D'Orb. Prod, de Pal^ont., t. ii, p. 389, No. 1019. 

 Leda deltoidea. Morris. Catal. Brit. Foss., p. 205, 1854. 

 Trigonoccelia deltoidea? Desk. An. sans Vert, du Bass, de Par., t. i, p. 840, 1858. 



Spec. Char. T. testa trigond vel deltoidea, tumidd, crassiusculd, subaquilaterali, con- 

 centrice striata ,• pedi-regione rotundatd, obsolete radiatd ; siphoni-regione angulatd, et 

 carinatd ; umbonibus magnis prominentibus ; cardine arcuato, dentibus 5 vel 6 utroque 

 latere ; fossuld connexus profunda, triangulari. 



Shell trigonal, tumid, rather thick, nearly equilateral ; concentrically striated ; pedal 

 region rounded and obsoletely rayed ; siphonal region angulated and keeled ; beaks large 

 and prominent ; hinge-line curved, furnished with 5 or 6 denticles on each side j depres- 

 sion for connector deep and triangular. 



Length, T 3 6 ths inch ; height, ith inch. 



Localities. Barton {Edwards), Hordle (S. Wood), Shapley Heath {Morris). 



