RETICULARIA 427 



Genus RETICULARIA McCoy 



Description. Shells small, of medium size or larger, subovate, subcir- 

 cular or subelliptical in outline, the hinge-line shorter than the greatest 

 width of the shell, the cardinal extremities rounded, the fold and sinus 

 moderately developed or essentially obsolete. The pedicle valve with a 

 rather small, arched cardinal area whose margins are usually ill-defined, 

 the surface usually rounding with but little or no interruption into the 

 lateral slopes of the valve; the delthyrium rather large and open. Inter- 

 nally the dental lamella? are strong and elongate sometimes extending an- 

 teriorly along the floor of the valve for more than one-third its length, 

 they may be nearly parallel or moderately divergent, between them, in the 

 median line of the valve, is a strong median septum which commonly 

 reaches further anteriorly than the dental lamellae, the muscular area sub- 

 rhomboidal in outline, usually not deeply impressed. The brachial valve 

 less convex than the pedicle, with narrow cardinal area; internally the 

 brachidium is simfilar to that of Spirifer, so far as known, the muscular 

 area is elongate ovate in outline and faintly impressed, often scarcely 

 recognizable, it is divided longitudinally by a median septum of greater 

 or less strength, which is sometimes almost obsolete. The surface of both 

 valves is covered by regular, concentric rows of fine spines, which are in 

 the form of double tubes, the position of these double tubed spines is 

 clearly shown upon the exfoliated shell layers and sometimes upon the 

 casts of the interior of the shells, but the tubes do not penetrate the full 

 thickness of the shell. 



Remarks. These shells have often been considered as being not gener- 

 ically distinct from Spirifer, but their surface markings are so very differ- 

 ent from members of that genus that most writers of recent years have 

 given the group full generic rank. The genus has sometimes been made 

 to include a group of species from the Pennsylvanian and Permian faunas 

 which possess similar surface markings of concentric rows of double tubed 

 spines, but which differ from these Mississippian shells in the entire ab- 

 sence of septal lamellae in either valve. It has been pointed out by Girty, 1 

 however, that the genotype of Reticularia was one of these septate shells, 

 and the non-septate form has been referred by the same author to the 

 genus Squamularia Gemmellaro. All the Mississippian species which have 

 been observed possess the internal septal plates and must be referred to 

 Reticularia. 



1 Garb. Form, and Faunas of Colorado, p. 387. (1903.) 



