BRACHIOPODA 



173 



Orbiculoidea. Ventral valve flattened ; on the surface just 

 behind the apex is a narrow furrow, which is perforated at the point 

 farthest from the apex, the perforation passing through the shell 

 obliquely backwards. Ordovician to Carboniferous, perhaps also 

 Mesozoic. Ex. 0. morrisi, Wenlock Limestone. 



Crania (tig. 77). Shell calcareous, traversed by vertical canals 

 which branch near the outer surface ; quadrangular or sub-circular, 

 smooth or with radiating ribs, fixed by the ventral valve ; without 

 peduncle-opening. Ventral valve depressed-conical ; dorsal larger 

 than the ventral, conical with a sub-central apex. Interior of each 

 valve with a border covered with granulations. Two pairs of well- 

 marked adductor impressions in each valve (a, a') ; the posterior pair 



.a, .... 

 r- 



a- 



Fig. 77. Crania anomala, Recent. A, interior of ventral valve; B, dorsal 

 valve, a, anterior adductors; a', posterior adductors; c, posterior 

 adjustors ; c', cardinal muscle ; r, o, central and external adjustors. 

 (From Woodward.) x 2. 



near the margin, the anterior near the centres of the valves and 

 close together, especially so in the ventral valve ; also other smaller 

 muscular impressions. A triangular protuberance near the centre 

 of the ventral valve. Ordovician to present day. Ex. C. ignaber- 

 ge?isis, Chalk. 



CLASS II. ARTICULATA 



The valves articulate by means of two teeth on the 

 ventral valve which fit into sockets on the dorsal. The 

 intestine is short and ends blindly. A brachial skeleton 

 may or may not be present. 



