GENERA OF THE SUBORDER ORTHOIDEA 



57 



enveloping the adductor ones. The individual adduc- 

 tor scars, when visible, are semi-elliptical, forming in 

 the aggregate a lanceolate impression. Usually the 

 adjustor scars are not clearly visible, being lodged on 

 the sides of the dental plates or as narrow tracks at 

 their bases. Ventral pallial sinuses usually consist of 

 two abbreviated trunks only, extending from the front 

 of the diductor impressions, except in Mimella, which 

 has a very remarkable system of sinuses. 



In the dorsal interior the structure is remarkably 

 uniform throughout the family. The cardinal process, 

 with its compressed and crenulated myophore, is uni- 

 form except in Platystrophui, where it is commonly 

 reduced almost to disappearance. However, the closely 

 related M cewanella has the typical cardinal process of 

 the family. 



The divergent brachiophores are moderately long 

 and bluntly pointed. They are supported by sharply 

 defined, convergent plates that unite with the floor of 

 the valve at the cardinal process, forming in some forms 

 a sessile cruralium; in a few specimens the supporting 

 plates actually unite with a median septum but this is 

 rare. 



In this family as elsewhere the mantle deposits extra 

 testaceous shell over the whole internal surface. This 

 tendency is particularly noticeable in Platystrofhia, and 

 it has been carried so far in the average forms of this 

 genus as to obscure the actual morphology completely 

 and cloud the genetic relationships. In the ventral 

 valve the dental plates are obliterated by deposition in 

 the umbonal cavities, and in the dorsal valve the fulcral 

 plates ma)' be lost from view by filling of the cavity 

 between that plate and the brachiophore support. 



The Plectorthidas form a homoeomorphic series with 

 Schizophoria of the Schizophoriids. Schizophorella 

 and Mimella of the former group represent the bicon- 

 vex or lenticular phase in the evolution, corresponding 

 to P'ionodema of the Schizophoriidas, while Hebertella 

 is the advanced or convexo-concave stage comparing 

 .well with Schizophorui. The evolution of the Hebert- 

 ellas is far more accelerated than that of the impunc- 

 tate group. The former are last seen in the Upper 

 Ordovician of North America, but Schizophoria persists 

 into Upper Pennsylvanian time. 



Subfamily PLECTORTHIN^ Schuchert 1929, 

 emended 



Coarse- and fine-ribbed Plectorthid.-c having wide 

 hinge-lines and unequal interareas. 



Discussion. — In emending this subfamily it is found 

 necessary to exclude from this association Eridorthis 

 and Cycloccclia. On a later page we have shown that 

 the former, by its internal structure, belongs to the 

 Glyptorthinae of tlie Orthidas ; and Cyclocaelia is aber- 

 rant in form and placed by us in a separate subfamily. 



On the other hand, we have transferred to the Plector- 

 thin.X' the genera Hebertella and Schizophorella. As 

 now constituted, therefore, the Plectorthinns form the 

 largest subfamily of the Plectortliidas, including the 

 following genera: 



Plectorthis Hall and Clarke 

 Hebertella Hall and Clarke 

 Mimella Cooper 

 Schizophorella Reed 

 Doleroides Cooper 



The genetic relationships are thought to be as shown 

 in Table 4. 



Genus PLECTORTHIS Hall and Clarke 1892 



PL 11, figs. 1-5, 9, 15 



Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, pp. 194, 

 221, pi. 5, figs. 18-20. 



Genotype. — Orthis plicatella Hall 1847, Pal. 

 N. Y., vol. 1, p. 122, pi. 32, fig. 9. 



Description. Exterior. — Transversely semiellip- 

 tical, anterior and lateral margins convex; lateral pro- 

 file lenticular, uncommonly with the ventral valve 

 concave at the front; anterior commissure faintly sul- 

 cate or uniplicate; interarea of the ventral valve 

 strongly apsacline ; slightly curved, comparatively short ; 

 dorsal interarea orthocline or faintly anacline, shorter 

 than the ventral interarea, slightly curved ; delthyrium 

 and notothyrium unmodified; ornamentation pauci- 

 costate to multicostate, interspaces marked by fine ele- 

 vated lines of growth. Test thin, fibrous, impunctate. 



Ventral interior. — Delthyrial cavity shallow, um- 

 bonal cavities deep, teeth small, situated on the apex 

 formed by the hinge- and delthyrial margins; crural 

 fossettes oblique, faintly defined; dental plates thin, 

 clearly defined, advancing, continued as a slight thick- 

 ening for some distance around the lateral margins of 

 the diductors. Muscle area heart-shaped, occupying 

 between one-third and one-half the length of the shell; 

 adductor track linear, confined within the diductor 

 scars; diductors subcresccntic; a low ridge extends 

 forward for a short distance from the adductor track 

 in some specimens; anterior margin and whole interior 

 multicostate. 



Dorsal interior. — Notothyrial cavity shallow; car- 

 dinalia confined to about one-fifth the length of the 

 shell; brachiophores short, stout, grooved inside, form- 

 ing the margin of the notothyrium and supported by 

 thin plates that converge and unite with the floor of the 

 valve beneath the cardinal process. Sockets deep, ex- 

 cavated beneath the palintrope, defined by a small ful- 

 cral plate. The brachiophore supports in old shells are 

 commonly obliterated by the deposition of adventitious 

 shell in the lateral cavities. Cardinal process a thick, 

 rounded ridge, having a crenulated myophore. In old 

 shells a short median ridge extends from the anterior 



