THE BRITISH FOSSIL BRACHIOPODA. 325 



mass, of a cylindrical form, with one extremity erected through the foramen of the ventral 

 valve, and the other resting within its umbonal region ; the protruded extremity is 

 protected by a thick horn-like covering of brownish colour. 



Dr. Van Bemmelen says that " the so-termed tendinous portions of the muscles were 

 found to be specially developed portions of the mesenchymatic layer under the real 

 insertions of the muscular fibres. This was most obviously shown in the occlusor 

 muscles of Waldheimia, where the tendons are united, to a considerable extent, with 

 the body-wall, and where in transverse sections, not the slightest difference or limit 

 between them is to be found. The same origin must be attributed to the peduncle, 

 which, contrary to that of Lingula, consists of a solid mass of mesenchymatic tissue 

 containing many fibres ; such fibres are also found on the margin of the mantle and the 

 free or inner walls of its tissues. They serve for support, and were believed to be 

 muscular by Hancock." 



The peduncle varies in length in different species ; in some forms it is so exceedingly 

 short that the beak becomes worn from its restricted movements in close contiguity to 

 the rock to which it is attached ; while in some species, such as some Lingula^ it often 

 attains and sometimes exceeds half a foot in length. It is sometimes found preserved in 

 the fossil condition, as I have shown to be the case in a specimen of Lingula Lesneuri 

 discovered by Mr. Vicary, as well as in an example of Eichwaldia {E. subtrigonalis, 

 Billings). Figures of these will be found in the pages of my ' Silurian Monograph ' and 

 of its Supplement. 



Dr. Van Bemmelen states (' An. Nat. Hist.,' 5th series, vol. xi, p. 382) that he found 

 the structure of the muscles to agree with the description Hancock had given of them. 

 " They consist of thin longitudinal fibres, perfectly parallel, and probably as long as the 

 whole muscle itself. Opposed externally to these fibres are found nuclei surrounded 

 by a very minute quantity of protoplasm. From this fact, he deduces the epithelial 

 character of the muscles, which probably have originated from the coelomic epithelium, 

 and, in becoming independent of it, have retained the nuclei of their formative cells. 

 All the muscular fibres were found to be smooth, with the exception of those of the 

 posterior occlusors, which are distinctly striated. In this fact, already mentioned by 

 Hancock, there is seen a new proof for the assertion of the Hertwigs, that between 

 smooth and striated muscles there need not be any morphological, but only a physiological 

 difference. The occlusores, posteriores and anteriores have the same function ; they are 

 inserted on the same tendinous mass ; but the former are striated, the latter smooth." 



Such is the general arrangement of the shell-muscles in the division composing the 

 Articulated Brachiopoda, making allowance for certain unimportant modifications observable 

 in the shells composing the different families thereof. Owing to the strong and tight 

 interlocking of the valves by the means of curved teeth and sockets most of the Brachio- 

 poda could open their valves but very slightly. In some species, however, such as 

 T/iecidium, as Lacaze-Duthiers informs us, the animal could raise its dorsal valve at right 



