IGO DR. T. DAVIDSON ON KECEXT BliACHIOPODA. 



each side of the central septum (Plate XXIII. fig. 15, c) and Liave their outer end attached 

 to the extremity of the cardinal process of the dorsal valve. M. Lacaze-Duthiers has had 

 the opportunity of studying a vast numher of individuals of this species of Thecidiiiia in 

 the living state ; and he mentions that, contrary to what we find to be the case in Tei^e- 

 hraiula, the animal opens its shell very widely — the dorsal valve rising on its hinge at 

 right angles to the ventral valve, like the lid of a snuff-box (woodcut, fig. 16, c). The 

 animal is also sensible to light and darkness ; and it draws down its smaller valve with 

 the rapidity of lightning on the approach of danger. 



M. Lacaze-Duthiers further observes that the mantle is exceedingly thin, and is not 

 famished round the margin with any of the long setse which exist in Terebratula and 

 L'mgida, and that the genital lobe (the one which corresponds to the concave or ventral 

 valve) differs much from the other, and is characterized in its thickest lobe, or central 

 portion (that is to say, towards the deep and concave j)ortion of the valve), by very thick 

 calcareous plates, the analogues of the plates and spicula occurring in the mantle of Tere- 

 hratula. Tiiese plates in Thecidium participate in the character of the shell itself, being 

 smooth on the under surface, and covered on the npperside with asperities similar to those 

 which cover the entire surface of the bottom of the valve. The plates are very thick, and 

 form a ceiling or vault over the cavity which contains the organs of reproduccion. Tliey 

 have been carefully described and figured by M. J. Bosquet, M. E. Deslongchamps, and 

 myself, as seen in several fossil species, and in particular in Thecidium cermlculare, from 

 the Upper Cretaceous beds of the Duchy of Limbourg. 



M. E. Deslongchamps says, at pp. 30, 31 of his memoir on the Organization of the 

 Mantle in the Articulate Brachiopoda: — In short, the Thecidiidte possess an altogether 

 peculiar organization of the mantle. Calcareous spicules crowding the external laminae 

 are no longer present, but true calcareous plates which obliterate almost the whole 

 interior, and form, as it were, a second shell, line the interior of the valves, and 

 give them a most strange and elegant appearance. We will not now repeat the details 

 of the conformation of these curious lamina? constituting the pallial apparatus, which 

 can, it would seem, be better studied in the fossil than in the recent species, and of which 

 a large number of authors have given excellent descriptions. We will only mention that 

 this apparatus undergoes very great modifications in different species, being sometimes 



excessively complicated, as in T. recurvlrostrls, whilst in others this apparatus is 



reduced to an indistinctly marked single lamella, as in T. Perrleri 



Another very striking peculiarity is revealed to us in the organization of the tissues of 

 Tliecidea medUerranea. We have seen hitherto, except hiArglope, which formed the ex- 

 ception, that it was in the arms and their cirri that the calcareous spicula wevQ especially 

 developed, and that it was in these organs that they began to appear in the earliest stages of 

 the shells, extending progressively on the two laminae of the mantle. We note that precisely 

 the contrary occurs among the Tliecididae. The majitle is no longer, so to say, merely a 

 calcareous mass, and the arms and their cirri do not show the smallest trace of spicula or 



even of calcareous granulations It is apparent that the arms constitute a simple 



diaphanous membrane, which is rather difficult to distinguish even by a feeble magnifying- 



