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 Zimopsis, but there is no extended area or bipartite 
division of the connector, as in that genus. 
M. Nyst proposed the name Zrigonocelia for those bivalves which resembled Pectun- 
culus, but differed in the disposition of the connector, as before remarked. Zrigonocelia, 
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 Zrigonocelia by M. Nyst. 
The present genus is an emanation from Zimopsis, differing from it by the loss of the 
expanded ligamental area, approaching closely to Zeda 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. Trigonocetia pevtoipEa? Lamarck, Tab. XIX, fig. 11, a—e. 
Nucuta DELTOIDEA? Lamk. Ann. du Mus., t. vi, p. 126, and t. ix, pl. 18, fig. 5. 
= —_ J. Sowerby. Min. Conch., t. 554, fig. 1. 
Limvopsis DELTOIDEA ? D’Oré. Prod. de Paléont., t. ii, p. 389, No. 1019. 
Lepa DELTOIDEA. Morris. Catal. Brit. Foss., p. 205, 1854. 
TRIGONOC@LIA DELTOIDEA? Desh. An. sans Vert. du Bass. de Par., t. i, p. 840, 1858. 
Spec. Char. T. testa trigond vel deltoidea, tumidd, crassiusculd, subequilaterali, con- 
centricé striatd ; pedi-regione rotundatd, obsolete radiata ; siphoni-regione angulatd, et 
carinatd ; umbonibus magnis prominentibus ; cardine arcuato, dentibus 5 vel 6 utroque 
latere ; fossuld conneawis profundd, 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 ; depres- 
sion for connector deep and triangular. 
Length, ths inch ; height, 4th inch. 
Localities. Barton (Zdwards), Hordle (S. Wood), Shapley Heath (Morris). 
