144 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



with the result that the muscle area is notably ex- 

 panded in front and is a trapezium in outline. The 

 median ridge carrying the adductor scars varies from 

 thin to very wide and in a few forms (S. lowensls, 

 S. striatula) is extended forward beyond the anterior 

 margin of the muscle area. In some specimens of 

 Schizophoria from the Pennsylvanian the median ridge 

 has attained the profile and height seen in Orthotichia. 

 In the ventral valves of S. resufinata, there are two 

 palHal sinuses extending forward from the anterior 

 ends of the diductors and bowing outward in a con- 

 siderable curve. 



In the dorsal valve there are some notable variations 

 in the musculature and pallial markings. In speci- 

 mens of the genotype, S. resufinata, the arrangement 

 of the adductor scars is nearly identical to that seen in 

 Orthotichia, with the exception that the anterior scars 

 are not in contact and that the space between the 

 longitudinal sets is not so great as in the latter genus. 

 In the early Schizophorias the muscle field is compact 

 and the right and left sets of adductors are in close 

 proximity, being separated only by a low median ridge, 

 but in the later forms the longitudinal sets migrate 

 laterally so that there is a notable space between them 

 occupied only by a remnant of the formerly large 

 median ridge. This lateral spreading of the two longi- 

 tudinal sets of muscles reaches its maximum in Orthoti- 

 chia. The generic difference between Orthotichia and 

 Schizophoria, therefore, is certainly not very great. 



The pallial markings are usually well developed, 

 especially in the dorsal valve ; S. macjarlani shows the 

 common type. This species has four parallel trunks 

 extending anteriorly from the front margin of the 

 anterior adductors to the shell margin, scarcely branch- 

 ing during their passage. Rarely there are six of these 

 trunks, the two outside ones corresponding to the 

 lateral trunks of horthis. So deep are these sinuses in 

 S. macjarlani that they have impressed themselves 

 deeply in the shell, so that the slightest exfoliation of 

 the shell laminze shows these sinuses as ridges. The 

 so-called S. profinqua (Hall) has pallial markings of 

 the horthis type and has accordingly been placed with 

 that genus. 



The earliest undoubted Schizophoria is S, senecta 

 from the Clinton of New York and probably also from 

 Port Daniel, Quebec. Therefore the group must have 

 its inception in the Ordovician, in Pionodema, whose 

 external resemblance to Schizophoria has already been 

 remarked upon. Furthermore, in the ventral valve of 

 Pionodema there is an incipient adductor ridge and the 

 structure of the cardinalia in the dorsal valve is identical 

 in every fundamental detail with that of Schizophoria. 



Genus ORTHOTICHIA Hall and Clarke 1892 

 PI. 24, figs. 12, 15,22-24,27 



Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 213, 

 pi. 7, figs. 11-15. 



Genoholotype. — Orthis ? morganiana Derby 

 1874, Bull. Cornell Univ., vol. 1, p. 29, pi. 3, figs. 

 1-9, 11, 34, pi. 4, figs. 6, 14, 15. 



Description. Exterior. — Externally like Schizo- 

 phoria but usually more finely ornamented. 



Ventral interior. — Umbonal cavity deep ; teeth 

 strong; dental plates strong, divergent, extended 

 nearly to the middle of the valve as high septal ridges 

 about the lateral margins of the muscle area; the 

 cavity defined by these plates is centrally divided by a 

 median septum, which takes its origin a short distance 

 in front of the apex, rising to its maximum height 

 a little in front of the ends of the dental plates, where 

 it is abruptly truncated. Diductor and adjustor scars 

 confined to the areas between the dental plates and 

 the median septum; adductor scars attached to the 

 sides of the median septum. 



Dorsal interior. — Cardinalia strong; brachiophores 

 strong, tusklike, supported by strong plates that diverge 

 around the lateral margins of the muscular area; all 

 of these structures are intimately fused together; the 

 sockets are deep and are defined by a thin concave ful- 

 cral plate which unites the crural apparatus to the 

 palintrope. Cardinal process small, "multifid"; muscle 

 area quadripartite, the anterior adductors subtriangular 

 or irregularly oval, situated so that their antero-median 

 extremities are in contact, the posterior extremities 

 being divergent and separated by the expanded ex- 

 tremity of the low median ridge; the anterior adduc- 

 tor scars on the outside of and slightly posterior to the 

 anterior pair and separated from them by a narrow, 

 curved, oblique ridge. Anterior scars narrow sub- 

 parallel grooves separated by a low ridge. Pallial 

 markings as in Schizophoria. 



Geologic range. — Pennsylvanian to Permian. 

 Distribution world wide. 



American Species 



Orthotichia morganiana (Derby) 1874 

 O. schuchertensis Girty 1903 

 O.kozLowskii King 1930 (= 1931) 



? O. texana Girty 1928 



Foreign Species 



Orthotichia chekiangensis Chao 1927 



O. free hi Fliegel 



O.indica (Waagen) 1884 



O. marmorea (Waagen) 1884 



O. morganiana (Derby) 1874 



O. morganiana chihsiaensis Chao 1927 



Distinguishing characters. — Orthotichia is dis- 

 tinguished by the presence of three strong septal plates 

 in the ventral valve, the outer two being the divergent 

 dental plates; the inner or median ridge rises to a crest 

 at the front of the muscle area as in Enteletes and is 

 obliquely truncated. This genus most closely resembles 



