12 BRITISH CARBONIFEROUS BRACHIOPODA. 



Spec. Char. Shell of an elongated, oval, or obscurely pentagonal shape, more or less 

 truncated in front, tapering at the beaks, and widest towards its middle or frontal region ; 

 valves almost equally convex, rarely gibbous, with a wide mesial depression or concavity 

 towards the front in the larger number of full grown individuals ; beak moderately pro- 

 duced, and but slightly incurved ; foramen rather large, oval, and in adult shells approxi- 

 mating the umbone of the smaller valve, so as to conceal much of the deltidium which 

 partly surrounds the aperture ; the lateral portions of the beak are somewhat flattened, the 

 ridges being indistinctly defined ; the lateral margin of the ventral valve indents the 

 opposite one in the proximity of the beak, and is straight or slightly raised in front ; 

 external surface smooth, marked only by a few concentric lines of growth ; shell structure 

 minutely perforated. In the interior of the ventral valve there exists two short, diverging, 

 dental or rostral shelly plates, while in the interior of the dorsal one a short, simple loop 

 is observable, occupying about one third or less of the length of the valve. Dimensions 

 variable. 



Length 24, width 18^, depth 13 lines. 

 „ 22± „ 14, „ 10 lines. 



Obs. M. De Verneuil and several other authors are of opinion that T. hastata 

 should be considered but as a variety of T. sacculus of Martin. This view was also 

 advocated, in 1843, by Professor De Koninck, but since abandoned from having observed 

 differences which appeared to him incompatible with the idea that both should be con- 

 founded under a single denomination. In his opinion, as well as in that of Professor 

 M'Coy, there exists two adult types, the one being infinitely larger than the other, " which 

 is proved by the thickening of the margin at or under one inch in length (T. sacculus), 

 the deeper notch in front, and the mesial sulcus existing at a much smaller distance from 

 the beak than in T. hastata ; and, above all, the species are distinguished by the great 

 difference of angle at which the valves meet at the margin, the front and lateral margin of 

 T. hastata being sharp and wedge-like, but those of Sp. sacculus being extremely blunt 

 and obtuse." Such are the characters by which Professor M'Coy distinguishes the two 

 shells ; but I confess that at times, and before a large series of both, one feels tempted to 

 look upon these differences as of little value, and to consider the one as a variety of the 

 other. A point worthy of notice is, however, that in no example of the true T. sacculus 

 have we observed any trace of those stripes due to colour which are so beautifully pre- 

 served in many specimens of Sowerby's shell, such as in those we have illustrated from 

 Derbyshire (figs. 6, 8, 9, 16). It is the opinion of Professor De Koninck and of 

 other authors, that the shell under description may have been intended for " Anomia 

 attenuata," Martin, 1 which was simply characterised by a few vague Latin words ; but as 



1 Martin, while alluding to species unfigured in his work, states that "five belong to the Perforati, 

 Faro.//, but only one there is any danger of mistaking for Sacculus, it may be distinguished by the 

 following characters : Conch. Anomites attenuatus. — C. anomites longitudinaliter ovatus laevis compressus, 

 margine acuto integerrimo." 



