32 BRITISH OOLITIC AND LIASIC BRACHIOPODA. 



shell, when, turning suddenly backwards by a rapid curve, it produces a considerable angle 

 to the first portion of the line and acute frontal depression. 



Loop in smaller valve simply attached to the crura, and extending to near the frontal 

 margin ; the inner side of the lamella, facing the opening of the valves, is irregularly 

 covered with spines, similar to those observable on the spine of Sp. rostratus and loop of 

 Ter. pectunculoides, &c. ; the outer side facing the sides of the valve is smooth, and always 

 without spines. 



Valves smooth, finely punctuated, and marked by numerous lines of growth. Dimen- 

 sions and form variable; length 16, breadth 14, depth 12 lines. 



05s. Many shells have been attributed to Sowerby's Ter. resupinata, apparently not 

 belonging to the type of that author, established on a basic shell, answering to the above 

 description. It is, however, no easy matter, indeed scarcely possible, to give an accurate 

 description of any species agreeing with all the shells it includes, from the innumerable 

 variations they constantly present, especially in some species which pass into one another 

 in the most perplexing manner ; so much so, that certain characters are diminished in 

 value from their irregularity. It is indispensable to give distinct names to certain forms, 

 Avhich in reality merge from one type into another, but would be incomprehensible or 

 difficult to remember, if we were compelled to give a long and interminable list of varieties, 

 with distinct denominations added to the typical name. The Resupinata group, for instance, 

 presents endless variety in form, most difficult to characterise, and especially so when we 

 have before us a great number of specimens. Sowerby's type is distinguished from the 

 other members of the group by its small elevated and recurved beak, diminutive foramen, 

 and strongly laterally compressed and keeled rostral valve, more especially so towards the 

 beak, which character seems peculiar to some of the liasic species. The depression and 

 longitudinal groove of the imperforated valve, which is strongly marked in this form, 

 varies to so great a degree, that, on a large assemblage of specimens, we trace pas- 

 sages into the next species or variety, Ter. Moorei, where, instead of the depression 

 visible in the type Ter. resupinata, we find the imperforated valve completely convex and 

 gibbous ; so much so, that the longitudinal furrow disappears, and, becoming sensible only 

 towards the frontal portion ; the larger valve presents likewise a slight longitudinal depres- 

 sion towards the front, corresponding with that visible in the other valve, never seen in 

 Sowerby's type of resupinata ; the margin line of the two valves is also nearly straight all 

 round, so that it would be difficult to recognise the original type of Ter. resupinata in 

 Ter. Moorei, had we not before us a number of specimens illustrating the gradual passage. 

 Ter. carinata, Ter. iiupressa, and other forms, which, though nearly allied and belonging 

 to the same group, should at least, for convenience, be considered as specifically distinct, as 

 they in general vary enough to be easily distinguished. It is to Mr. Moore we owe the 

 discovery of the first specimen, showing spines on the inner side of the calcareous loop, 

 facing the frontal opening of the shell, and which I subsequently recognised to be peculiar 

 to several species. Ter. resupinata is abundantly found in the marlstone beds of the 



