﻿130 BRITISH FOSSIL TRIGONIJi;. 



rows of costa3 upon the sides of the valves are numerous (24 to 27), narrow, and much 

 elevated, passing from the area obliquely downwards and forwards, large in their middle 

 portions and attenuated at their two extremities ; nearly straight upon the anterior face, 

 they disappear at the border, their other extremities form a slight undulation as they 

 approach the area. The few last-formed or posteal costae are small, narrow, nearly 

 pei-pendicular, or slightly waved; their lower extremities project, forming an irregular 

 dentated lower border ; their upper extremities are united to the transverse costellse 

 on the area. All the costa3 have narrow, fringing, obtuse, nodose elevations, more or 

 less irregular in their size and prominence, minute and evanescent near to the extremities 

 of the rows. The lines of growth are conspicuous on the anteal face of the shell, and 

 in well-preserved specimens each of the intercostal spaces has a small median elevated 

 line ; frequently, however, this feature is only obscurely indicated. The typical form 

 occurs in the Neocomian formation ; delicately preserved examples are obtained in the 

 Isle of Wight at Atherfield in the beds called " Crackei's," and also in other beds and 

 localities in a less favorable condition of preservation. Few of the specimens exceed two 

 inches in length, measured upon the area. Internal moulds, which probably belong to 

 T. caudata, are smooth, and their borders are without indentations ; their apices are more 

 erect than in specimens with the tests preserved. 



British specimens differ materially from the figures of this species given in the 

 ' Paleontologie Fran^aise ' by D'Orbigny, where a specimen about two inches in length 

 with eighteen costas upon the sides of the valves is represented with an area which 

 retains bounding narrow ridges and regular transverse costellse throughout its entire 

 length, or, in other words, the peculiarities of the immature state are continued in the 

 more advanced stage of growth, an abnormal condition to which we find no approxi- 

 mation. 



The specimens figured by Agassiz are all apparently immature, and are only moulds 

 upon which some portions of the surface characters are visible. His description is 

 limited in accordance with such unsatisfactory materials. 



Stratigraphical position and Localities. — The Cracker beds of the Neocomian 

 formation at Atherfield have yielded it rather abundantly. 



Foreign Localities. — France : Bettancourt, Auxerre, Saint Saveur, Comble, Morteau. 

 Switzerland : Neuchatel. 



Trigonia scabricola, Li)c. Plate XXVII, figs. 4, 5, 5 a, 5 b. 



Tkigonia scabra, Morris. Catal., p. 229, 1854. (Non T. scabra, Lam.) 



This large and abundant Trigonia of the Blackdown and Ilaldou Greensand has 

 usually been referred to either T. scabra or T. caudata; to the latter species it is 



