74 



INTRODUCTION. 



Shell transversely or longitudinally oval ; valves unequally convex ; beak acute, entire, 

 and slightly incurved ; area defined and divided in young individuals by a large triangular 

 fissure, which is afterwards reduced to an oval foramen, entirely surrounded by a delti- 

 dium, and finally closed in some adult individuals; external surface smooth; valves 

 articulating by the means of a large tooth situated on either side of the deltidium, close 

 to the hinge line, and fitting into corresponding sockets in the smaller valve. In the 

 Fjg 24. Fig. 25. interior of the ventral valve a large 



mesial longitudinal septum extends 

 to a short distance of the frontal mar- 

 gin : this plate is thick at its origin 

 and base, but gradually decreases in 

 width while increasing in depth, as 

 it recedes from near the extremity of 

 the beak. In the smaller or dorsal 

 valve, a massive curved cardinal pro- 

 cess stretches to the opposite valve 

 where it clasps the ventral mesial 

 septum with its bifurcated ex- 

 tremity ; near the base of this pro- 

 cess in the dorsal valve a longi- 

 tudinal septum arises (smaller than 

 that of the ventral valve), and which 

 divides the quadruple impressions of the adductor muscle ; the socket walls are very much 

 expanded, forming prominent hinge-plates curving inwards on each side of the cardinal 

 process. The lower portion of the hinge-plate supports the crura of the loop in the shape 

 of two flattened stems or lamellae, which after proceeding with a slight upward curve to 

 near the extremity of the septum, are suddenly reflected, and again approach the sockets 

 before sweeping sub-marginally round in the shape of a large wide loop, from the inner 

 edge of which a number of smaller lamellae branch off" and converge. 



Ohs. Great efforts have repeatedly been made to obtain a complete knowledge of the 

 internal characters of this most remarkable shell, so solidly constructed, and so dissimilar 

 from what we observe in the other sections of the class.^ Some of the internal arrange- 

 ments have been known for many years, since we find an attempt to illustrate the large 



Sfringocephalus Burtini.^ 



24. Inferior of the dorsal valve, partly restored, from ii specimen in 



the Collection of Professor King. 



25. A section of the valves; j, cardinal process or boss; s, septum in 



dorsal valve; vs, septum in ventral valve; t, sockets; c, eitira of 

 the loop; 1, loop; a, adductor. 



' I feel greatly indebted to Professor King for the communication of the original specimen from which 

 his diagram ('English Permian Fossils,' pi. xix, fig. 1, 1849), had been drawn; figures 24 and 25 

 restored from the Professor's specimen, but want the branch lamellae discovered by M. Suess. I am likewise 

 greatly indebted to MM. de Koninck, Beyrich, Bouchard, and Suess for the communication of specimens 

 and sketches of their best examples of the genus. 



^ Baron von Buch describes at great length the external appearance of this shell, but makes no 

 allusions to any of its internal characters, he places it, with some doubts, among the Terebratulce. 



