part 2] SY1UNG0THYBI8 AND SPIBIFEBIKA. 209 



The plications of the shell impart to the anterior margin a charac- 

 teristic serrate appearance. 



' The surface of the shell is marked by a few concentric imbri- 

 cating lamella; of growth, widely spaced in the umbonal region, 

 but closely crowded anteriorly. In well-preserved specimens the 

 outer layer is covered with fine, tubular, spinose projections, which 

 when broken, as is usually the case, give to the shell a character- 

 istic pustulose appearance. 



The area. — The area of the pedicle-valve is variable in height, 

 but the average apical angle is 140°. When the area is low it is 

 strongly incurved, though as its height increases it becomes flatter ; 

 the plane of the area is always inclined to the plane of the 

 brachial valve, so that the apex of the pedicle-valve is above 

 that of the opposite valve. The area is defined by a pair of lines 

 diverging from the apex, and outside of which the shell gradually 

 curves over into the lateral slopes. 



The delthyrial angle is about 40°. In his original description 

 ITOrbigny stated that the ' delthyrial aperture ' was without 

 covering ; but this was due to the imperfection of his specimens. 

 The delthyrial covering consists of a pair of imperforate deltidial 

 plates slightly inclined one to the other to form a roof-like structure. 

 The delthyrial covering is rarely preserved; but its former existence 

 is indicated by the grooved margins of the fissure. 



Shell-structure. — The shell is fibrous and strongly punctate: 

 that is, perforated by comparatively-large canals which appear to 

 push the fibres aside. The canals communicate with the interior 

 of the shell, so that the surface of well-preserved internal casts is 

 granulated. 



Internal structures : — Pedicle -valve. — The delthyrial sup- 

 porting-plates are well developed, diverge towards the floor of the 

 valve, and their anterior extremities are concave. The ventral 

 median septum is well developed. It is attached to the floor of the 

 valve for about a third of the distance between the apex and the 

 anterior margin ; near the apex it extends to the level of the area, 

 and it terminates anteriorly in a curved blade-like process. 



An apical callosity is developed to a variable but never very great 

 extent, and although the extreme apical portion of the valve is 

 solid, the callosity has but little influence on the internal cast. 



The spiral coils in the brachial valve are large, and have their 

 apices directed laterally ; their anterior portion is spinose. The 

 jugum is a complete simple transverse band, connecting the primary 

 lamella 1 of the spires near their junction with the crura. 



The essential characters of Spirifirina. as portrayed by the 

 Liassic species, may be summarized as follows : — Hinge-line 

 not greater than the diameter of the shell; cardinal extremities 

 rounded ; area moderately low and curved ; distinct incurved 

 beak; few angular costa> ; angular fold and sinus not sharply 

 differentiated from the costae ; well-developed ventral median 

 septum ; coarsely -punctate shell-structure. 



The possession of the two last-named characters is not alone 

 sufficient to justify the reference of a shell to Spiriferina. 



