THE BRITISH FOSSIL BRACHIOPODA. 295 



posed of keratode with a subordinate portion of phosphatic calcareous matter. The 

 shell-structure of Lingula and Discina has been likewise studied by Dr. Gratiolet in his 

 admirable memoir, " Etudes anatomiques sur la Lingula anatina.' ! Therein he mentions 

 that " the structure of the valves in the Lingulida appear at first sight to differ much 

 from that of the Terebratulida, but a more attentive examination reveals analogies at first 

 sight unexpected. Two distinct elements exist in the shell of Lingula, the one corneous and 

 the other testaceous ; they occur in thick layers alternating from the convex to the concave 

 surface of the valves, commencing with a superficial corneous one. These layers are not, 

 however, of equal thickness throughout. On the convex side the thickness of the horny 

 layers is greater towards the outer surface of the shell, while on the visceral or inner surface, 

 the testaceous elements predominate. They are of especial thickness at the level of the pos- 

 terior angle of the rhomb. The thick testaceous layers are separated by thin corneous ones 

 which thin out at certain points. This arrangement gives an amount of opaqueness to the 

 central portions of the valves, while the outer edges, where the horny substance predomi- 

 nates, are semitransparent. The structure of the testaceous layers simulates that of the 

 Terebratulidcs ; they are crossed by numerous microscopic canals, which are traversed by 

 extremely delicate striae, recalling the chains formed by the prismatic elements of the shell 

 in the Terebratulidce. This description applies equally to Discina, but in this last the 

 calcareous predominates greatly over the horny portion." 



When alluding to the shell-structure of Crania Mr. E. Deslongchamps says that it 

 is peculiar " that the valves are in reality formed of two fitting layers, the upper one 

 being compact, while the lower one, on the contrary, shows us a multitude of horizontal 

 striae of an extreme delicacy, which are no doubt due to an excessive division of the 

 fibrous element. These striae are crossed by a great number of small canals, which are 

 disposed in an irregular manner and do not pass through the entire thickness of the valve, 

 but terminate in a point at their extremity. To this arrangement of shell-structure he 

 gives the name of channelled limestone (calcaire canicule)." 2 



Dr. Johan T. Van Bemmelen in two very able memoirs, 8 published in 1882 and 

 1883, "On the Anatomy of the Brachiopoda," described and illustrated with great 

 minuteness the structure of the shell ; and he tells me that his results have been almost 

 the same as those attained by Hancock. In his paper he says that the structure of the 

 shell was investigated by himself by making transverse sections through decalcified frag- 

 ments. " No communication," he adds, " between caecal cavities in the mantle-tubules 

 and lacunae or vessels in the mantle itself was found. The corpuscles in these tubules 

 seemed to be for the greater part nuclei belonging to cells that clothe the walls of the 

 shell-perforations. No openings in the periostracum occurred on the tops of the caeca, 

 which tops showed with perfect clearness the radiating ring of fine striations discovered 



1 'Journal de Conchyliologie,' tome viii, p. 49, 1860. 



2 ' Paleontologie Frai^aise, Brachiopodes Jurassiques,' t. vi, pp. 17, 18, PI. viii, 1862. 



3 ' Annals and Mag. of Nat. Hist.,' 5th series, vol. ii, p. 3/9, 1883. 



