174 



BRACHIOPODA 



ORDER I. PROTREMATA 



A pseudo-deltidium is developed, but sometimes dis- 

 appears in the adult. The peduncle-opening is at the 

 margin of the ventral valve, in the form of a fissure 

 (delthyrium) either entirely open or more or less completely 

 closed by the pseudo-deltidium. Usually there is no 

 brachial skeleton. This group is found mainly in the 

 Palaeozoic formations ; the only living form is Thecidea 

 {Lacazella). E.g. Ovthis. 



Kutorgina. Shell calcareous, broader than long, with a long, 

 straight hinge-line ; surface with concentric striae. Ventral valve 

 very convex, with an elevated umbo ; four pairs of muscular im- 

 pressions. Dorsal valve flat or slightly convex, with two pairs of 

 muscular impressions. Areas of both valves narrow, with a wide 

 fissure. Hinge rudimentary. Cambrian. Ex. K. cingulata. 



Fig. 78. Productus giganteus, Carboniferous Limestone. A, interior of 

 dorsal valve ; B, interior of ventral valve ; C, ideal section of both 

 valves ; D, dorsal hinge-line. ;', cardinal process ; a, adductor ; 

 r, divaricator; h, ventral area ; b, brachial prominence (?) ; s, hollows 

 occupied by the spiral arms ; v, reniform impressions. (From 

 Woodward.) x $. 



