140 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



aberrant Enteleti'nae this ornamentation is superim- 

 posed over broad undulations or plicae. Shell fibrous, 

 punctate. 



The ventral interior is especially marked by promi- 

 nent diductor scars, separated by a median ridge which 

 becomes more and more elevated in the more advanced 

 and geologically more recent genera. The dorsal 

 cardinalia consist of tusklike brachiophores having 

 widely divergent supporting plates; the cardinal process 

 is typically dalmanelloid; the dorsal adductor scars 

 are separated by an oblique ridge in the later forms. 

 The cardinalia, being alike throughout the family, are 

 its most significant feature. The dorsal pallial sinuses 

 consist usually of three pairs; the two inner ones, 

 given off at the anterior ends of the anterior adductor 

 scars, bifurcate immediately at their inception and ex- 



Table 13 



Enteletinae 



Schizophoriinae ■ 



-> Isorthina: 



tend to the front margin as four subparallel trunks. 

 The third pair is given off from the oblique ridge sepa- 

 rating the adductor scars and extends to the front 

 margin nearly parallel to the other sinuses. The inner 

 four trunks are usually the more prominent. 



Geologic range. — Ordovician (Chazy) to 

 Permian. 



The Schizophoriidas are subdivided into the follow- 

 ing subfamilies: 



Schizophoriinffi Schuchert 



Enteletinse Waagen 



Isorthinas Schuchert and Cooper 



The genetic relations appear to be as shown in 

 Table 13. 



Discussion. — The Schizophoriidas is one of the 

 most easily recognized and longest lived families of 

 the Dalmanellacea. It has its inception in the early 

 Middle Ordovician (Chazy) with Pionodema as the 

 oldest representative of the family, starting with the 

 biconvex or lenticular shell phase. The passage from 

 Pionodema to Schizophoria appears to be almost im- 

 perceptible. In Pionodema the ventral septum is low 

 and long, being of the euseptoid type (Fredericks), 

 and is the place of attachment of the adductor muscles. 

 This septum increases in height during geological time 

 and attains its final development in Orthotichia and 



Enteletes. In Pionodema the adductor scars are 

 clearly visible on the median ridge, and although in 

 the geologically higher forms no adductor scars are 

 visible on the floor of the valve, the presumption is 

 that they were also borne on the septum. 



The cardinalia are nearly uniform throughout the 

 Schizophoriidae from Pionodetna to the last of the 

 Enteletes in the Permian. The brachiophores are 

 always shaped like the tusk of a boar, sharp and curved. 

 The supporting plates are widely divergent and al- 

 ways have a small fulcral plate defining the socket. 

 The cardinalia are perhaps the most distinctive struc- 

 tures in the whole group and their uniformity from 

 genus to genus throughout the range of the family is 

 a rather definite expression of the value of the cardinalia 

 as a family characteristic. 



The modification of the dorsal musculature from 

 Pionodema to Orthotichia has some value in chrono- 

 logical stratigraphy. In Pionodema the adductor scars 

 have a quadripartite arrangement, the posterior scars 

 directly in back of the anterior pair and separated by 

 horizontal ridges. Later, however, in Devonian and 

 Mississippian times, the posterior adductors have migrat- 

 ed laterally and taken a position outside of and posterior 

 to the anterior pair. These scars, further, are rather 

 oval in outline instead of subcircular or quadrate as in 

 the earlier forms, and are separated by an oblique 

 ridge. The two oval scars one above the other pro- 

 duce a subflabellate appearance much like that of the 

 ventral valve of Rhifidomella. The lateral migration 

 of the posterior scars leaves a large central area occu- 

 pied only by the much reduced median septum. The 

 higher, more specialized forms such as Orthotichia, 

 Enteletes, Parenteletes, etc., continue the dorsal mus- 

 culature of the later Schizophorias. 



Subfamily SCHIZOPHORIIN^ Schuchert 1929 



Lenticular, or convexo-concave Schizophoriidae not 

 having numerous strong undulations or plicae developed 

 at the front end of the shell. 



Geologic range. — Middle Ordovician (Chazy) 

 to Permian. 



Embraces the following genera: 



Pionodema Foerste 

 Schizophoria King 

 Orthotichia Hall and Clarke 

 Aulacophoria Schuchert and Cooper 



The genetic relations are thought to be as shown in 

 Table 14. 



In this subfamily we place Pionodema as the origi- 

 nating stock of the Schizophoriidae, this genus giving 

 rise to Schizophoria out of which came the culminat- 

 ing Orthotichia. Pionodema and Schizophoria form 

 interesting homoeomorphs with Schizophorella and 



