480 



^BORTT- FIFTH REPORT ON THE STATE MUSEUM. 



sion to the object of support. Among the edentulous genera 

 there is rarely the least indication of such an area. In Acrotreta, 

 Conotreta, Acrothele, Iphidea, etc., where the foramen is apical 

 there is a posterior flattening of the pedicle- valve, upon which 

 the concentric ornamentation of the shell is retained. This has 

 sometimes been inaccurately termed a " false cardinal area," or a 

 " pseudo-area." Though not so sharply defined as the cardinal 

 area in the articulate brachiopods, it is nevertheless an homolo- 

 gous structure and needs no other designation. In such genera 

 as Discwisca, Orbiculoidea, etc., there was no obstruction to the 

 posterior growth of the shell, and therefore no cardinal area. 



TheDeltkyrium and its Coverings. 

 By the term delthyrium is meant the median triangular 

 fissure or cleft which crosses the cardinal area of the pedi- 

 cle valve ; or when that area does not exist, traverses 



Fig 53.— Pedicle passage 

 of Schizocrania Jilosa. 

 F, triangular pedicle 

 fissure; g, listrum. 



Fig. 54.— Tr emails treminalis. 

 Interior of pedicle-valve. F. 

 foramen; v, vascular sinu- 

 ses; </, pedicle-groove. 



the posterior surface from apex to cardinal line. It is 

 the passage for the pedicle, and always subserves this function 

 when existing in an uncovered condition in adolescent or mature 

 growth. In Orthis this is its normal condition at maturity ; but 

 it has been shown that the open delthyrium at this stage, in 

 Orthis varica, is actually a modified phase, the shell having 

 assumed in youthful growth a convex covering or deltidium, 

 which is subsequently resorbed. In Sohizocrania the triangular 

 slit is retained in all stages of growth, and in Trematis its outer 

 edges converge at maturity, while in < Orbiculoidea it is altogel her 

 a feature of early growth, the acjult shell having the passage 

 closed by the union of the edges on the posterior margin, and 

 the contemporaneous formation of a transverse plate or listrium 

 beneath the apex. In the inarticulate genera [phedea and Voir 



32 



