﻿SCABBY. 



115 



§ VI. SCABRiE. 



This, the predominating section of the genus in the Cretaceous rocks, is also special 

 to them ; the entire series has crenulated or scabrous costse and costellse ; the area and 

 escutcheon, devoid of bounding carinas, are each limited by an angularity of the surface ; 

 the transverse costellse of the escutcheon for the most part also pass across the area. It 

 is decisively separated from all other sections by the position of the ligamental fissure, 

 which is strictly inter-umbonal, so that in some species when the valves are united the 

 ligament is concealed, or when the umbones are not closely placed the lengthened 

 narrow fissure extends anteally to them. 



In Britain the Scabrte form three sub-groups, which may be designated the aliformis, 

 the pennata, and the spinosa groups. The first of these is remarkable for the prolonged 

 and attenuated posteal extremity ; the siphonal border is, therefore, very short ; the 

 incurrent orifice, the excurrent and anal orifices, were arranged near to each other, but 

 separated by a short internal rib, as in other examples of the genus ; in Britain its 

 representatives are T, aliformis, T. Vectiana, T. caudata, T. scabricola, T. Elheridgei, T. 

 Meyeri, and T. Fittoni ; there are also a numerous series of foreign analogous forms. 



The second or pennata group has the attenuated posteal extremity of the first 

 group ; it is characterised by a feature which somewhat resembles the section of the 

 Glabra ; it has an ante-carinal diagonal depressed space, which is either smooth or has a 

 faintly defined series of perpendicular costse. T. pennata and T. sulcataria are the only 

 British species ; two other species occur in France. 



The third or spinosa group has the borders of the valves comparatively short and 

 rounded. The pallial costas and costellas of the area and escutcheon diverge from the 

 angle of the valve. T. spinosa, T. ornata, T. Archiaciana, T. Agassizii, T. Vicaryana, and 

 T. Cunningtoni are its British representatives. The foreign analogues of this group are 

 even more numerous. 



A fourth group, which does not appear to be represented in Britain, is characterised 

 by possessing the usual ornamented escutcheon of the Scabra together with the area of 

 the Clavellata, with their bounding carinas and delicate transverse plications; the pallial 

 costse are extremely variable. T. Lusitanica, Sharpe, T. elegans, Baily, and T. Con- 

 stanta, D'Orbigny, may be referred to this group. 



