GENERA OF THE SUBORDER ORTHOIDEA 



93 



Ptychopleurella matapedia, ii. sp. 



PI. 6, figs. 2, 5 



Shell small, subrectangular, latcr.il margins nearly 

 straight, anterior margin rounded, probably emargi- 

 nateT Hinge-line straight, equal to the greatest width 

 of the shell. Anterior commissure sulcate. Ventral 

 valve subpyramidal, interarea long and broad, strongly 

 apsacline, beak not incurved, delthyrium narrov/, tri- 

 angular. Dorsal valve convex, the greatest convexity 

 in the vicinity of the umbo. There are eight primary 

 costx on the dorsal valve, the middle two elevated 

 above their fellows and defining a deep sulcus. There 

 are nine secondary costae, two occupying the sulcus. 

 The latter two take their origin about 1 mm. from the 

 beak. Ventral v.-ilve provided with eight primary ribs 

 and ten secondaries. The median ridge is depressed 

 below the surface of the ventral sulcus. Each primary 

 rib, except those at the interarea borders, has two im- 

 planted parasitical secondary ribs on either side, thus 

 forming fascicles of three ribs. At the front of the 

 shell there are three ruga; to a millimeter. 



Dimensions (holotype) : 



Width at Width 



Length widest part Thickness at hinge 



6 mm. 8.5 mm. 4.5 mm. 8.5 mm. 



The nearest relative of this species is P. bouchardi, 

 but the Canadian form differs chiefly in its wider 

 hinge-line, that of P. bouchardi being much narrower 

 than the total width of the shell. Other important 

 differences from P. bouchardi are seen in the narrower, 

 more elevated ribs and lesser rugae. 



Horizon and locality. — Lower Devonian ? or 

 highest Silurian, 3 miles east of Dawsonville, New 

 Brunswick, on the Upsalquitch road, Quebec, Canada. 

 Holotype, Cat. No. 287, Schuchert Collection, Yale 

 University. 



Family DINORTHIDyE Schuchert and 

 Cooper 1931 



Progressive Orthacea probably derived out of the 

 Orthidse and having a subquadrate ventral muscle 

 field, Orthis-likt brachiophores, and a simple cardinal 

 process with a crenulated myophore. A prolific group 

 characteristic of the Ordovician. 



The family embraces the following genera: 



Dinorthis Hall and Clarke and its subgenera, as 

 follows : 



Dinorthis s. s. 



Pltpsiomys Hall and Clarke 

 Pionorthis Schuchert and Cooper 

 Retrorsirostra Schuchert and Cooper 

 Marionella Bancroft 



Valcourca Raymond 



Multicostflla Schuchert and Cooper 



Auitinclla Foerste 



Planidorsa Schuchert and Cooper 



? Palirostrophomena Holtedahl 



Discussion. — Externally most of the Dinorthidx 

 are convexo-concave in outline and rather easily recog- 

 nized because of their strophomcnoid habit. The ven- 

 tral muscle field is unique for its very large adjustor 

 scars, the expansion of which makes the subquadrate 

 field so characteristic of the family. A small apical 

 deltidium is not infrequent. 



In the dorsal valve the brachiophores are essentially 

 of the Orthis type, but the cardinal process is progres- 

 sive, since it is crenulated on its posterior surface for 

 the attachment of the diductor muscles. 



In view of our reduction of Pltrsiomys to the status 

 of a subgenus of Dinorthis, it is no longer desirable to 

 continue the subfamily name Plssiomiinse of Schuchert 

 1913^^ and his recasting of it in 1929'^* to include 

 what are now seen to be unrelated genera. 



The genetic relations of the Dinorthids are thought 

 to be as shown in Table 6. 



Genus DINORTHIS Hall and Clarke 1892 



PI. 9, figs. 1-11, 13, 14, 20-23; pi. 10, figs. 15, 17, 

 18,24-26; t. figs. 5, 18 



Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, pp. 195, 

 222, pi. 5, figs. 27-33. 



Genotype. — Orthis fectinella Emmons 1842, 

 Geol. N. Y., Rept. 2d Dist., p. 394, fig. 2. 



This genus is here divided into five subgenera: (1) 

 Dinorthis s. s. Hall and Clarke; (2) Pltesiomys Hall 

 and Clarke; (3) Retrorsirostra Schuchert and Cooper; 

 (4) Pionorthis Schuchert and Cooper; (5) Marion- 

 ella Bancroft, which appears to be a parallel develop- 

 ment with Pleesiomys. 



Description (of dinorthis sensu latissimo). 

 Exterior. — Shells transversely semielliptical to subquad- 

 rate, lateral profile convexo-concave to unequally 

 biconvex; anterior commissure rectimarginate, faintly 

 sulcate or uniplicate ; ventral valve commonly broadly 

 sulcate ; dorsal valve frequently sulcate but the depres- 

 sion always narrow and shallow; ventral interarea 

 longer than the dorsal, apsacline; dorsal interarea ap- 

 sacline or orthocline; delthyrium generally open, but 

 more uncommonly partially closed by a short deltid- 

 ium; notothyrium partially closed by cardinal process. 

 Ornamentation pauci- to multicostate, and with fine 

 concentric elevated growth-lines and lamellx of 

 growth. Test fibrous, impunctatc, with occasional 

 tubulose costellae. 



"" Zittel-Eastman Text-book of Paleontology, 2d ed., 

 vol. I, p. 382. 



"Foss. Cat., Pars 42, p. 14. 



