THECIDEA. 13 



I 



wise ornamented, granulated, structure punctuated ; hinge line more or less straight, with 



two strong teeth in the attached valve, adapting themselves into corresponding sockets in 

 the smaller valve ; beak more or less produced, with long or wide well-defined area and 

 deltideum. Interior of valves variable ; in larger valves a longitudinal, central, and two 

 lateral ridges are generally more or less visible, under which two deep muscular impres- 

 sions are seen ; upper valve complicated, more or less deeply and regularly sinuated by an 

 apophysary testaceous ridge, united all round, and leaving a small cavity in the upper 

 portion of the valve free for the body of the animal, these sinuated ridges varying in 

 number, position, and extent in different species ; two strong lateral adductor muscles 

 situated under the hinge, no arms, animal small. 



Obs. We are acquainted with five British Oolitic species of Thecidea, four from the 

 Lias, a fact hitherto unrecorded, and two in the Inferior Oolite. 



7. Thecidea Moorei, Bav. Plate I, fig. 10. 



Diagnosis. Shell irregular, inequivalved, attached by the greatest portion of its 

 inferior valve, almost square; attached or lower valve, modelling itself to the object on 

 which it is fixed, with elevated sides and front rising perpendicularly from the attached 

 part ; area well defined, triangular, and receding from its junction with the upper valve ; 

 deltidium large, elevated with slight central depression, and marked, as well as the area, 

 with numerous lines of growth. Upper valve almost flat, of a transversely oblong square, 

 with slight depression in the centre, and the sides, except at the hinge, turning sharply 

 over, and forming elevated sides, till they meet the edge of the lower valve, so that the 

 front is very much elevated. Surface strongly punctuated ; interior of attached valve only 

 known ; hinge line straight, with two strong teeth and elevated mesial ridge. Length 

 and breadth about 2 lines ; frontal elevation 1 line. 



Obs. Twelve specimens of this remarkable little Thecidea were found attached to a 

 specimen of Rhynchonella serrata, from the marlstone or middle lias in the neighbourhood 

 of Ilminster, along with Thecidea Bouchardii and triangularis, by Mr. Moore, to whom 

 I feel much pleasure in dedicating the species. These and another, T. rustica, are the first 

 specimens of this genus hitherto noticed, as far down as the Liasic period ; no mention is 

 made of this genus in M. D'Orbigny's Prodrome ; they would appear to be the oldest 

 Thecideas at present known. Mr. Moore also has one specimen of this species, found in 

 the upper lias, from the neighbourhood of Ilminster. TJi. Moorei is easily distinguished 

 from all other known Thecideas, by its square shape and elevated front. 



Plate I, fig. 10, illustrates specimens of natural size, from the collection of Mr. Moore; 

 fig. 10, a — e, are enlarged. 



