DE. T. DAVIDSON ON EECENT BRACHIOPODA. 



20 & 



Fis. 21. 



Mu^iiified section 

 thickest portion 

 shell of Limjida ana 

 titta (uftcr(iratiolct). 



of 

 of 



A. Horny layer. 



B. Calcareous layer, per- 

 forated hy canals. 



in layers or thin laminae, which succeed each other alternately from the convex to the 

 concave surface of the valves after a superficial layer, Avhicli is horny. The layers do 

 not present anywhere the same thickness ; on the convex face of the 

 shell the thickness of the horny layer is the largest, on the inner or 

 visceral face the calcareous layers are the thickest ; those thick 

 testaceous layers are separated hy thin horny layers, the shell 

 being more semiti'anspareut where the horny layers prevail. The 

 structure of the lioruy layers is very simple ; they are transparent, 

 yellowish, passing into green in certain species, and appeared to me 

 formed of parallel fibres without any trace of perforations. The 

 structure of the calcareous layers recalls that of the Terebratulidse. 

 They are traversed by a multitude of microscopic canals, and are 

 likewise traversed by striti; of an extreme delicacy. The internal 

 surface of the valves is lined by a very dry and very thin mem- 

 brane, which can be detached without tearing away at the same time 

 portions of the very thin testaceous laminte. M. S. Cloez has 

 prepared an analysis of the test of Lingula, which has been 

 published in the ' Comptes E-endus de la Societe Philomatique ' of 

 Paris ; and this author remarks that the composition of the shell 

 resembles that which M. Chevreul has shown, in a work published 

 some years ago by Ilatchard, to exist in the scales of the Lepidostrees 

 and insects, and that the great proportion of phosphate of lime in 

 the shell of the living LinguJce deserves the attention of zoologists and geologists, and 

 gives a great interest to the results furnished by microscopic analysis." 



Space unfortunately will not allow of my entering into details in connection Avitli the 

 anatomy of the animal of Lbujnla anatlna, which would require many elaborate illustra- 

 tions. I must therefore refer the student to the special works upon the subject abo^e 

 quoted. 



Cuvier, in 1797, gives tlie first very brief account of the animal witli which we are 

 acquainted. He represents the two lobes of the mantle, one of which he has partly 

 bent backwards, so as to expose the spirally coiled labial appendages. He describes 

 also the alimentary canal, mouth, and anus ; and in 1802 he published a more elaborate 

 account of the animal accompanied by thirteen figures. 



Hancock states, in his memoir on the " Organization of the Brachiopoda " (Phil. Trans. 

 E-oy. Soc. cxlviii. 1858, p. 794), that in Lingula " the body of the animal is depressed, and 

 occupies a much larger portion of the shell-cavity than it does in either the Terebratulidce 

 or BhyiichonelUdce. The pallial lobes are rather stout ; but the great sinuses are never- 

 theless distinctly seen through their membranes ; they are not united behind, as in 

 the articulated Braehiopods, but are free, and extend some distance from the body 

 all round the posterior or umbonal region. The body thus becomes well defined, the 

 lateral walls beino? at ri"-ht ambles to the dorsal and ventral. The two latter are verv 



COO • 



delicate, and so transparent that the viscera are quite apparent through them, tlie liver 

 and genitalia being the most conspicuous. The lateral parietes are strong and muscular, 



28* 



