TEREBRIROSTRA. 31 



Plate IV, figs. 4 and 4 °. Interior of smaller valve, illustrating the arrangement of the 



apophysary system, from a specimen obtained near Valognes, 

 and preserved in the collection of M. Deshayes in Paris, 

 (enlarged.) 

 „ fig. 4 s . The dental valve, also enlarged. 



11. Trigonosemus incerta, Dav. Plate IV, fig. 5. 



Diagnosis. Shell of an elongated oval shape, longer than wide ; valves almost equally 

 convex : beak produced, rounded, moderately recurved, and truncated by a small oval 

 foramen ; area triangular, nearly flat, short, deltideum edging a small portion of the 

 foramen ; surface of valves ornamented by about thirty-four rounded costse ; the greater 

 number arising from intercalation ; these are closely intersected by numerous small and 

 close concentric lines of growth. Structure punctuated ; length 4^, width 4, depth 2^ lines. 



Obs. This beautiful little shell was discovered by Mr. Moore in the Chalk with green 

 grains of chard, where it appears very rare. It is always unsafe to establish a species 

 on the inspection of a single specimen, which is all we have been able to obtain of the 

 present form ; but the shell seems adult, and differs much in general aspect from Trigo- 

 nosemus elegans, being more oval, and ornamented by fewer and stronger costae, as well as 

 by the almost equal convexity of its valves, and straight frontal line ; the area and beak 

 are shorter, and the foramen comparatively larger. I have therefore temporarily named it 

 T. incerta, hoping that the discovery of other specimens may enable Palasontologists to fix 

 its real place with greater certainty. 



Plate IV, fig. 5. Specimen, nat. size, from the collection of Mr. Moore. 

 „ fig. 5" Jc . Enlarged illustration of the same. 



Sub-Genus — Terebrirostra, D'Orbigny. 1847. 



Shell generally elongated, more or less oval, inequivalve, the dental valve much longer 

 than the smaller or socket one, extending in the shape of a long beak, with a flat, false 

 area and narrow deltideum ; the foramen truncating the extremity of the beak, and 

 partially completed by the deltideum ; hinge articulating by means of two teeth in larger 

 and corresponding condyles in smaller valve; internal disposition of the calcareous supports 

 unknown ; a mesial longitudinal plate is seen in the interior of smaller valve, probably 

 destined to support a doubly-attached loop ; structure punctuated. 



Obs. This sub-genus is at present characterised so very unsatisfactorily, from our igno- 

 rance as to its internal disposition, that it is with great difficulty I have been able to make 

 up my mind even to its temporal admission in this Monograph. I do so in hope that the 



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