﻿BRITISH FOSSIL TRIGONI^E. 



5tli. — Quadrate. 



Tlie QuaclratcB constitute a small section approximating to the ClaveUatcB, 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. dadalcu, Parkinson, T. Orhi(jniana,\s^c.\ the 

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

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

 ClaveUatcB. The interiors of the valves have, as in the Clavellatce and the Glabrce, a divi- 

 sional siphonal ridge. All the British Quadratce are Cretaceous. 



Cth. SCABR^. 



Unlike the Trigoni.'E 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 carinse 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 Scahrae have, towards their 

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

 from the incurrent respiratory canal ; there is also a lengthened scries 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. allformis. All the British Scahrcs, twelve in number, belong to the 

 Cretaceous Rocks ; they arc T. crenuJata, Lam., T. aliformis, Park., 1\ caudata, Ag., T. 

 Fittoni, Desh., T. Etherid^d, Lye, T. sjjinosa, Park., T. oniafa, U'Orb., T. Archlaciana, 

 D'Orb., T. Pideti, Coq., T. tenui-suhata, Duj., T. Pyrrlia, 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 



