INTRODUCTION. 11 



There is one other feature common to all the sections, but so much more strongly 

 denned in the Costata, that it may here be adverted to, more especially as it has not 

 previously been noticed. The oblique median furrow upon the area, forming the lower 

 border of the superior or more depressed portion of the area, indicates the position of an 

 internal rib ; in the living mollusk this rib constituted a short process only, which, near 

 to the outer border, served to separate the incurrent from the excurrent respiratory ori- 

 fice ; with progress of growth the process advanced continually, and its former position 

 was obliterated by the deposition of new-formed shell-substance j the external furrow 

 remained, and indicates the former position in growth of the internal divisional ridge. 



With the exception of Trigonia peninsularis, Coquand, which occurs in the Cretaceous 

 rocks of Spain, all the species of the Costata are Jurassic. The following thirteen species 

 occur in British strata: T. costata, Sow., T. denticulata, Ag., T. pulla, Sow., T. Meriani, 

 Ag., T. elongata, Sow., T. augustata, Lye, T. sculpta, Lye, T. monilifera, Ag., T. 

 Cassiope, D'Orb., T. Cruris, Lye, T. gregarea, Lye, T. hemispharica, Lye, T. tenui- 

 costa, Lye 



8th. — ByssifeRjE. 



I propose to constitute this section to include T. carinata, Ag., from the Neocomian beds 

 of France and England, a lengthened sub-cylindrical shell, which in general ornamen- 

 tation resembles the costata ; but in addition it acquired at the period of adult growth 

 a byssal aperture, formed by a slight excavation of the anterior border of each valve ; the 

 condition of the area and of the shell generally at that stage of growth indicates that it 

 became fixed or stationary. The elongated and almost cylindrical figure is very abnormal 

 as regards the genus Trigonia, but we perceive a strong resemblance to Bysso-arca and 

 to the relation which the latter holds to the genus Area ; the resemblance is much 

 enhanced by a general worn appearance of the upper surface in adult forms in both these 

 mollusks, which doubtless was the result of similar conditions of existence. The condi- 

 tion of the hinge has not been ascertained. 



General Sketch of the Distribution of the Genus Trigonia throughout the Geological 



Formations of Britain. 



The lowest geological position in which the genus occurs in Britain is in a single 

 stratum about the middle of the Upper Lias, exposed upon the coast scars of Yorkshire, 

 at the Peak, Robin Hood's Bay ; here T. litterata occurs in all conditions of growth ; 

 it is one of the Undulates. 



The Supra-liassic Sands of the Cotteswold Hills have produced four species very 

 sparingly, and a fifth occurs in a similar position at the Peak ; even thus early in the 



