112 



anteriorly produced, and unlike the rest of the Acephalse 

 directed forward ; the ligament which attaches the valves to each 

 other is external with a posterior narrow lunule. The surface of 

 the test which occupies its anteal portion is usually ornamented 

 either with ribs, tuburcles, or tuburculated ridges, on a different 

 plane to its posteal, which is well marked by an obliquely directed 

 ridge towards the posterior and lower extremity, a smaller area 

 called the escutcheon is bounded by another ridge, which includes 

 the ligamental plates, and ligament ; the hinge is complicated, 

 the teeth of both valves interlock each other, which, together with 

 its solidity, contributes to th perfect preservation of the shell in 

 its fossil state ; detached valves, or even the cast of an half-opened 

 shell is very rare. The right valve is furnished with two 

 prominent teeth, of which the posterior is directed backwards, 

 forming the figure of V with the other, which fits into two deep 

 sinuses of the left valve. The interior of the shell is smooth and 

 nacrous, shewing no indication of the external ornamentation 

 except in some few cases, where the tuburcles are feebly 

 represented. 



Casts which shew the interior of the shell are frequently met 

 with, but are of little use to the palaeontologist, for they cannot 

 with any certainty be discriminated from those of allied species. 

 One of the characteristics of these casts is a deep longitudinal 

 furrow, the wide gap intervening between the two distant beaks 

 correspond with the thick massive hinges ; the impression of the 

 posterior muscular scar is generally present. 



Agassiz divided the fossil Trigonise into seven sections, to which 

 Dr. Lycett in his magnificent monograph on the British Fossij 

 Trigonise published in the Palaeontographical Society has added 

 an eighth ; the distinctions are founded upon the shape of the 

 shell, the ornamentation of the surface, area, and its escutcheon. 

 Only four of the sections are represented in the Dorsetshire 

 formations, Clavellatce, Glalrce, Scabr<s, and Costatce. 



The surface of the valves of the section Clavellata are orna- 

 mented with tuburculated costse in concentric or oblique rows 

 the area is bounded by two tuburculated ridges, the escutcheon is 



