42 



DR. T. DAVIDSON ON EECENT BEACHIOPODA. 



& 702, 1880 ; E. Deslongchamps, Etudes critiques sur les Brachiopodes, pp. 172 & 177, 1881 ; Davidson, 

 Pal. Soc. Brit. Foss. Brach. vol. v. p. 323, 1884. 



Shell longer thau wide, ovate or subpentagonal, broadest about the middle, straight 

 in front, yellowish or light brown. Dorsal valve convex, sometimes flattened from 

 about the middle of the valve to the front ; ventral valve rather deeper and more convex 

 tlian the opposite one, with a slightly raised longitudinal flattened elevation or fold 

 along the middle. Beak moderately incurved, truncated by a circular foramen, some- 

 wliat separated from the hinge-line by adeltidium ; beak-ridges sharply defined. Surface 

 of both valves from the beaks generally smooth up to a certain age, then radiating and 

 irregularly plaited, some ribs being shorter than others, the central one straight, those on 

 tlu> lateral portions of the valves somewhat curved. Shell-structure perforated by canals. 

 In the interior of the dorsal valve the loop is long and simple ; after being attached to 

 the hinge-plate and its crural processes given off, the principal stems are outwardly 



Fis. 3. 



Fig. 4. 



Waldheimiajlavescens. Interior of both Valves, enlarged. 

 Fig. 3. Interior of the ventral valve : /, foramen ; d, deltidium : t, teeth ; a, adductor impressions (=occlusors 

 of Hancock); c, divaricators ( = cardinal muscles of King = muscles diducteurs principaux of Gratiolet) : 

 c , accessory divaricators ( = muscles diducteurs of Gratiolet) : h, ventral adjusters ( =ventral peduncular 

 muscles, or muscles du pedoncule, paire superieure, Gratiolet) ; 6', peduncular muscle. 



Fig. 4. Interior of the dorsal valve : c, c , cardinal process : h, h', hinge-plate ; «, dental sockets ; I, loop ; q, crura ; 

 .^ a, «,, adductor impressions ; m, septum. 



curved, and on reaching to about three fourths of the length of the valve become 

 reflected. The median septum extends along the bottom of the shell from under the 

 hinge-plate to a little beyond one third of the length of the valve. Brachial or labial 

 appendages largely developed and united to each other by a membrane. The principal 

 lateral branches commence on either side of the mouth, curving outward and facing 

 the bottom of the smaller valve, extend to within a short distance of the front ; then 

 becoming suddenly bent back upon themselves to within a short distance of the 

 mouth, are by an elegant semicircular curve directed towards the centre of the larger 



