GENERA OF THE SUBORDER ORTHOIDEA 



71 



Species 



Orthottrofhui canaJensis Clarke 1907 



O. djrta, n. sp. 



O. JL\om Focrste 1909 



O. newsomeiiiis Foerste 1909 



O. strofhomenoUes (Hall) 18S7 



Distinguishing characters. — The characteristic 

 features of Orthostroph'ui are the unequally biconvex 

 or convexo-concave profile, the small, deeply impressed 

 muscle field in the ventral valve, and the muscular 

 area of the dorsal valve which is elevated on a deposit 

 of adventitious shell. More fundamental than all of 

 these, however, are the pallial and ovarian impressions 

 described in detail below. 



Discussion. — The simple ventral musculature and 

 the cardinalia in Orthostroph'ui seem to relate this 

 genus most closely to Orthis, Hesferorthis, and their 

 allies. Confined to a short and deeply impressed del- 

 thyrial cavity, the individual muscle-scars are not 

 strongly impressed on the shell and in some specimens 

 they can not be individualized. When the separate 

 scars are visible, only two sets can be seen. The 

 median or adductor track is triangular, rounded and 

 wide in front. The "diductors" are narrow and sub- 

 crescentic. It is possible that the wide central track 

 is the equivalent of the combined adductors and diduc- 

 tors and that the scars usually referred to as diductors 

 are actually adjustor scars; we are not able to prove 

 this at present. 



The cardinalia appear to be orthoid. The brachio- 

 phores are simple processes supported by adventitious 

 shell, and the sockets are between the sloping outer 

 face of the brachiophore and the shell wall. The 

 cardinal process is a simple linear ridge, but in some 

 shells may be supplemented by oblique accessory eleva- 

 tions such as are seen commonly in Schixoramma. 



The dorsal musculature of Orthostrophia is decid- 

 edly orthoid in character but h.as the usual feature of 

 bipartite anterior adductors, discussed at length under 

 Productorthis and Schizophoria. 



The pallial markings have considerable resemblance 

 to those of Dinorthis (Pltpsiomys) and Mimella. 

 Their detail is described on an earlier page. The com- 

 bination of characters in Orthostrophia relate it to the 

 Orthidje and Plectorthidae, and it is placed tentatively 

 in the latter family, mainly on the basis of the pallial 

 marks. 



Orthostrophia dartae, n. sp. 



PI. 6, figs. 23, 31 



Shell small, thin, biconvex, subelliptical to subcir- 

 cular in outline. Ventral valve with a well defined 

 fold in the young stages which develops into a sulcus 

 at the front. In the dorsal valve there is a sulcus at 

 the beak and for half the length of the valve, which 

 then develops into a prominent fold produced beyond 



the anterior margin. Surface multicostellate, six or 

 seven costella; in 5 mm. at the front of the shell. 

 Ventral muscle field confined, about one-fifth the total 

 length of the shell. Adductor scars small. 



Measurements: 



Holotype 

 Paratype 



Length 

 19 mm. 

 IS 



Width 

 21 mm. 

 20 



This form occurs in the Middle Silurian of Port 

 Daniel, Gaspe Peninsula, Quebec. It is named for 

 Miss J. Doris Dart, who assisted Professor Schuchert 

 in making known the Silurian stratigraphy of the 

 scenic Port Daniel area. 



Family SKENIDIID^ Kozlowski 1929 



Small subpyramidal Orthacea probably derived out 

 of Finkelnburgta, having a spondylium, a cruralium, 

 and fulcral plates forming the sockets. 



Geologic range. — Middle Ordovician into Lower 

 Devonian. 



This small family embraces the genera Skenidio'tdes 

 Schuchert and Cooper and Skenidium Hall. At first 

 sight they appear to be tiny clitambonitoid shells and 

 best referred to the Clitambonacea, but the interior 

 characters of the dorsal valve appear to indicate that 

 they are more closely related to the Plectorthida or 

 even the older genus Finkelnburgia, hence we accept 

 the place given them by Kozlowski. 



The evolution of the family appears to be as follows : 



Skenidium 



.1.. 

 Skenidioides 



I 

 Finkelnburgia 



Genus SKENIDIOIDES Schuchert and Cooper 

 1931 



PI. 10, figs. 6, 8, 10-14 



Schuchert and Cooper, Amcr. Jour. Sci. (5), vol. 22, 1931, 

 p. 243. 



Genoholotype. — S. billingsi Schuchert and Cooper 

 1931. 



Description. Exterior. — Small, semi-elliptical, 

 hinge-line straight. Cardinal extremities acute or 

 nearly a right angle. Lateral profile unequally bicon- 

 vex, plano-convex, or slightly concavo-convex. An- 

 terior commissure sulcate; ventral fold gentle, round- 

 ed, sulcus deep or shallow. Ventral palintrope strongly 

 apsacline or catacline; beak may or may not be in- 

 curved; delthyrium open. Dorsal palintrope short, 

 anacline, notothyrium open. Surface multicostellate; 

 test fibrous, impunctate. 



