BRITISH FOSSIL TRIGONLE. 



5th. — Quadrate. 



The Quadrata constitute a small section approximating to the Clavettata, but distin- 

 guished by the shorter figure, by the more quadrate outline, by the very large flattened and 

 only slightly separated area, by the ornamented escutcheon, and by the great irregularity 

 and excentricity in the arrangement of the rows of tubercles or varices upon the sides of 

 the valves. T. rudis, Parkinson, may be taken for the best known British type of this sec- 

 tion, which also includes T. nodosa, Sow., T. dadalea, Parkinson, T. Orbigniana, Lye. ; the 

 latter, which has generally been mistaken for T. dadaiea, presents in its sub-ovate figure, 

 bi-partite area, and three nodose carinas, an approximation to or connecting link with the 

 Clavellatce. The interiors of the valves have, as in the Clavettata and the Glabra, a divi- 

 sional siphonal ridge. All the British Quadrates are Cretaceous. 



6th. — Scabile. 



Unlike the Trigoniae generally, the form is usually lunulate or crescentric rather 



than trigonal, but much inflated anteally ; the umbones are produced and recurved more 



than usual ; the superior or hinge-border is much excavated, the posterior extremity is 



produced, rostrated, and attenuated ; the area has almost disappeared, excepting in the 



adult condition, which has indications of bounding carinas towards the posteal portions of 



the valves ; the large upper surface in these crescentric forms is occupied almost solely by 



a great concavity, which represents the escutcheon, and which is ornamented by transverse 



costellae, similar in character to the costae upon the sides of the valves. The costated 



portion has the rows for the most part ridge-like and imperfectly tuberculated, or they are 



scabrous or serrated. The interiors of the valves in the Scabrce have, towards their 



attenuated posteal portions, a lengthened divisional ridge, which separated the excurrent 



from the incurrent respiratory canal ; there is also a lengthened series of small, regular, 



transverse, dental processes and alternate pits upon a narrow flattened plate that borders the 



escutcheon, its entire length in both the valves supporting an internal ligament or auxiliary 



portion of the hinge apparatus,and appears to be special to the present section in the Trigoniae, 



reminding us of a similar feature in the genus Leda. Examples will be given in figures of 



the interiors of T. aliformis. All the British Scabrce, twelve in number, belong to the 



Cretaceous Rocks ; they are T. crenidata, Lam., T. aliformis, Park., T. caudata, Ag., T. 



Fittoni, Desh., T. Etheridgei, Lye, T. spinosa, Park., T. ornata, D'Orb., T. Archiaciana, 



D'Orb., T. Picteti, Coq., T. tenui-sulcata, Duj., T. Pgrrha, D'Orb., T. Constantii, D'Orb. 



Of the foregoing, the first five only possess that remarkable elongation and attenuation 



of the posteal portion of the valves which tends to separate this section so prominently 



