RHYNCHONELLA. 77 



minutely striated, which is only seen on well-preserved specimens ; it is also a smaller species, 

 and found in the Oxford Clay, while B. acuta is peculiar to the Middle Lias. The difference 

 between the two forms has been noticed by M. Ch. Rouillier, 1 but we may here mention, 

 that a variety of Bh. cynocephala, found in the inferior Oolite of Minchinhampton, with 

 only one central elevated plait, somewhat approaches to B. acuta; but besides this shape 

 being unusual to B. cynocephala, its lateral costae are more numerous than those observable 

 in the Sower by shell. 



70. Rhynchonella cynocephala, Bichard, Sp. Plate XIV, figs. 10, 11, 12. 



Terebratula cynocephala, Richard, 1840. Bull, de la Soc. Gcol. de France, vol. xi, 



p. 263, pi. iii, fig. 5 abci . 



Diagnosis. Shell inequivalved, nearly as wide as long, more or less irregularly 

 triangular. The smaller or imperforated valve is convex at the umbo, and continues to 

 rise rapidly to the extremity of the margin, with a slight inward curve, forming a narrow, 

 pinched, elevated, and bidentated mesial fold, the sulcus separating the two plaits only 

 appearing in the anterior half of the valve, and increasing in width with little depth to the 

 edge of the front, to which a small plait corresponds in the sinus of the other valve. 



At a short distance from the umbo on either side of the elevated mesial fold, the sides 

 of the valves form a descending curve divided by four small lateral plaits on each side; the 

 beak in the larger valve is small, acute, not much recurved, leaving a well-defined false area 

 between the beak ridges and dental margin, in the centre of which, under the beak, a small 

 foramen is visible, surrounded by the deltidial plates, but almost touching the umbo in one 

 point; a regular convex curve is formed from the extremity of the beak to the front, which 

 is in a great measure taken up by a wide sinus ; in the middle of it is perceived the small plait 

 above noticed, four lateral costae also exist on both sides of this valve; they form a slight 

 outward curve at a short distance from the beak, and correspond with those in the smaller 

 valve. Structure imperforated. Length 8, width 8^, depth 7 lines. The width of the sinus 

 is about half the total breadth, the greatest width being also towards the middle of the shell. 



05s. Much confusion and misunderstanding exists relative to this species, which 

 has been considered by many the representative of Professor Phillips's Ter. bidens, c Geol. of 

 York.,' PL XIII, fig. 24 ; this is, however, an error, as the illustration given by that author 

 in no way represents the species under consideration, nor does the strata alluded to, 

 "Marlstone and Ironstone beds," allow us to mistake the shell of Professor Phillips for 

 B. cynocephala, which in England belongs to the Inferior Oolite. 2 In 1840, Mr. Richard 

 became convinced of this fact, which he then mentioned to me; and, in order to put matters 

 right, described and figured the present form under the name of T. cynocephala, from the vague 



1 Etudes Progressives sur la Palseontologie des Environs de Moscou. Par MM. Ch. Rouillier et Alex. 

 Vossinsky. Bull, de la Soc. Imperiale des Naturalistes de Moscou, 1847, No. 11. 



2 M. de Verneuil has found this species in the Lias of Villas des Covo and Albanaar in Spain. 



