﻿COSTAT^. 153 



considerably in their general outline, and less so in the prominence of their carina3 and 

 intercarinal costellae ; the latter are never large, usually delicate or minutely denticulated. 

 •Commonly there is no median carina excepting in the very young shell which usually has 

 the carinas and costellse strongly defined ; the area is also more concave. 



The acquisition of numerous well-preserved valves from the grey limestone (Inferior 

 'Oolite) of Cloughton, near Scarborough, has enabled me to compare and separate from them 

 without difficulty, a small, more narrow, costated form which occurs rather abundantly in 

 the Great Oolite of South Lincolnshire ; th(! valves have usually suffered compression and 

 are rarely well preserved ; a specimen in unusually good condition is figured, PL XXIX, 

 fig. 4. The costse are usually smaller and more numerous, the escutcheon more narrow, 

 and the hinge-border shorter than in the Inferior Oolite specimens of T. denticidata ; 

 they are equally distinct from other recognised species : upon the whole it seems proper to 

 arrange them as a variety of T. denticidata. 



JJinities and Differences. Agassiz described T. denticidata from a single specimen, 

 and expressed his indecision whether to regard it as a distinct species or only as a 

 variety of T. monilifera ; his figures of each of these species represent a single example of 

 immature form in which the characteristic features are but slightly developed, and T. 

 denticidata, although figured from a specimen in a fine condition of preservation, possesses 

 but little of the aspect exhibited in specimens of more advanced growth, which have less 

 general convexity, less prominence in their carinaj ; and their areas are less concave. 



T. mo7iiIifera, a much larger species, has its surface-ornaments altogether more 

 prominent ; its costae are larger and more distantly arranged ; the escutcheon more 

 especially has its surface-ornaments very distinct. 



T. pullus, a small species abundant in the Lower Oolites of Gloucestershire and 

 Wiltshire, has larger costas ; the surface-ornaments of the area are coarse and conspicuous; 

 the escutcheon is also especially distinct. 



T. scidpta, including its varieties, has greater convexity, the area much more coarsely 

 and prominently sculptured ; the simple flexure upwards of their costae, anteally, contrasts 

 with the undulation in T. denticidata. 



Trigonia costata differs from T. denticulata in the general form, which is more tri- 

 gonal, truncated, and erect ; in the conspicuous truncation of the anterior border ; in the 

 peculiar undulation of the costse ; in the more prominent area with its larger reticula- 

 tions ; it has also larger carinae, with more conspicuous indentations. 



Positions and Localities. T. denticulata appears to have a considerable and unusual 

 extent of stratigraphical range, if I am correct in placing with this species costated forms 

 nearly allied to each other, which occur in several widely separated horizons of the Lower 

 Oolites. Possessing little prominence in their characters as species, they have neverthe- 

 less much general resemblance, and are incapable of being clearly separated ; so that, as 

 compared with other forms, they may be distinguished from them chiefly by negative 

 characters only. The partial indecision which attaches to certain supposed examples of 



