78 INTRODUCTION. 



Obs. We are indebted to Defrance for the institution of this excellent genus/ which has 

 received the sanction of all naturalists ; but perhaps no section contains an assemblage of 

 shells more variable in their internal arrangements : these differences are chiefly observable 

 in the interior of the smaller valve. In some species, such as the Th. Moorei, Beslong- 

 champsii, Wetherelliit rusUcum, hippocrepis, sinuatum, &c., the granulated margin on reaching 

 the front extends inwards to a variable distance, assuming the shape of a narrow mesial 

 ridge or expanded concave and sinuated dissepiment ; in other forms, such as the Th. 

 radiatum, recurvirostris, Mediterraneum, &c. the mesial dissepiment is irregular in shape, 

 giving off at variable distances unequal lateral branches ; while in some cases, Th. 

 digitatum, hieroglyphicum, rugosum, (D'Orb.) &c., numerous dissepiments, variable in form 

 and dimensions, arise from all round the margin, into which the apophysary ridge extends 

 and follows the curves of the digitated grooves. Great dissimilarity likewise exists in the 

 extent of the attached surface of the dental valve, which has a great influence on the regu- 

 larity or shape of the species and valve ; but however great or small the affixed portion is, 

 it invariably originates from the extremity of the beak.^ 



The generality of species have both their valves convex, or the smaller one almost flat ; 

 but in Thecidia Leptanoides, (Deslong.) not only is the socket valve concave, but it is much 

 smaller than the other, which protrudes considerably beyond the margin of the ventral one. 

 Geol. Range: — 'This genus, so far as we know, first appeared in the Salt-marls of 

 St. Cassian.^ In the Lias, the species was both numerous and large, from that epoch the 

 genus continued uninterruptedly to the present period, and the section is still repre- 

 sented by a single species. 



Examples : Tli. Moorei, Dav.; Mayalis, Leptcenoides, sinuatum, Koninckii, E. Deslong. ;* 



^ Dictionnaire des Sciences Nat., vol. liii, 1828. 



2 M. D'Orbigny states in the fourth volume of his 'Paleont. Fran9. Ter. Cretaces,' p. 151, that 

 on examining the beak of some of the TerebratuUform species, he observed the remains of a foramen 

 whence a pedicle must have issued ; and in my second part, p. 13, I reproduced that view, but which I now 

 consider to be very improbable, and feel much more disposed to believe that Thecidium was in all cases, in 

 the young age, fixed by the substance of its beak to sub-marine bottoms. 



2 Klipstein describes and figures, in his 'Beitrage zur Geol. Kenntness der Astlichen Alpen,' plate xv, 

 fig. 19, a species which he terms Spirifer bidorsatus, but on an examination of the original figured example 

 now in the British Museum, Mr. Woodward pointed out to me that it most probably was a Thecidium 

 approaching in character to the Leptcenoides, Desl. It is certainly not a spirifer, and exhibits undeniable 

 traces of its attachment by a portion of its ventral valve. 



* In a very interesting and valuable paper on the Leptcenas and Thecidia lately discovered in the 

 Oolitic formations of Normandy, published by M. Eugene Deslongchamps, in the ninth volume of the 

 'M^moires de la Soc. Lin. de Normandie (1853),' the learned author adds several new and interesting 

 details relative to Thecidium, dividing the genus into two groups, from the presence or total absence of 

 what we have termed " the apophysary ridge,''' and I cannot do better than to add an extract in the 

 author's own words — 



" D'apres I'inspection des interieurs des dificrentes especes de Thecid^es de tons les terrains, je pense 

 qu'on pent diviser facilement ce genre en deux sections. Si Ton regarde avec attention les interieurs de 

 deux especes qui au premier coup- d' ceil, semblent etre assez voisines, la Thecidea Mayalis (esp^ce 



