496 FOSSIL BRACHIOPODA 



CHAP. 



to the continual pressure of the lobes of the liver. The division 

 of the umbonal cavity into definite chambers in Mono^nerella^ 

 and to a less extent in other members of this family, appears, 

 according to Davidson and King, to have been caused by pressure 

 of the ovarian lobes. 



In connexion with the foregoing remarks on the development 

 of the " platform," it may be mentioned that the paths along 

 which the muscle-bands move, as the shell of Brachiopods in- 

 creases in size, are marked by elongated scars, and often by 

 shelly deposits ; and when the members of a muscle-pair come 

 into juxtaposition these shelly deposits (which act as fulcra for 

 the muscles) combine, and by the growth of the shell form a 

 septum, as in the case of the median septum of Liyigulepis. 



The Oholidae show some important features in the internal 

 impressions. Oholella erassa (Hall) may be taken as a well- 

 known type of the famil}^ In this species a pair of small scars, 

 one on each side of the pedicle-groove, lies close under the 

 hinge line in the ventral valve. There is also a well-marked 

 scar for the insertion of the pedicle-muscle at the end of the 

 pedicle-groove. A pair of much elongated lateral impressions 

 extending forward from the " cardinals " may be homologous 

 with the "laterals" of Lingula; and the two small central scars 

 between them may be compared with the " centrals " of Liyigida 

 which are in a somewhat similar position. In the dorsal valve 

 of 0. erassa a pair of " cardinals " is found, and on each side of 

 a low median rounded ridge are two small " central " scars. 

 Indistinct " lateral " scars arise close to or in the central area, 

 and diverge anteriorly. 



Sometimes a great concentration of muscle-scars occurs 

 round the foramen in the ventral valve, as in Siplionotreta. 



As regards the minute structure and composition of the shell 

 in the Ecardines, we find that the Lingulidae and Discinidae 

 have their shell composed of alternating layers of phosphate of 

 lime and a corneous substance ; the former layers are pierced 

 by microscopic canals. Tlie Craiiiidae have calcareous shells 

 traversed by tubules, which divide into many fine branches near 

 the external surface ; a thin periostracum covers the exterior. 

 The Trimerellidae have heavy thick calcareous shells, for which 

 they required the previously-described elaborate arrangement of 

 muscles to open and shut them. 



