496 IFOSSIL BRACHIOPODA chap. 



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

 umbonal cavity into definite chambers in Monomerella, 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 increases 

 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 Lingidepis. 



The Obolidae show some important features in the internal 

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

 known type of the family. 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 Lingula which 

 are in a somewhat similar position. In the dorsal valve of 0. 

 crassa 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 Sijdionotreta. 



As regards the minute structure and composition of the shell 

 in the Ecardines, we find that the Lingtdidae and Discinidae 

 have their shell composed of alternating layers of phosphate of 

 lime and a corneous substance ; the former layers are pierced hj 

 microscopic canals. The Craniidae have calcareous shells tra- 

 versed 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. 



