GENERA OF THE SUBORDER ORTHOIDEA 



73 



Genus SKENIDIUM Hall 1860 



PI. 13, figs. 1-5,8 



Hall, 1 3th Rept. N. Y. State Cab., 1 860, p. 70, figs. 2, 4, 



5 (not 1, ?3). 

 Koilow-ski, Pal. Polonica, vol. 1, 1929, pp. 46-51. 



Genolectotype (Hall and Clarke). — Orthis in- 

 signis Hall 1859, Pal. N. Y., vol. 3, p. 173. 



Description. Exterior. — Shell small, subpyra- 

 midal, semicircular or semielliptical in outline; hinge- 

 line straight; cardinal extremities usually acutely an- 

 gular, not uncommonly subauriculate. Lateral profile 

 subtriangular; ventral valve triangular, gently convex; 

 dorsal valve gently convex or plane. Anterior com- 

 missure sulcate. Ventral palintrope long, strongly ap- 

 sacline to procline; dorsal palintrope nearly obsolete. 

 Surface ornamentation consisting of simple or dichot- 

 omous costae; test fibrous, impunctate. 



Ventral interior. — Like Skenidioides in possessing a 

 spondylium completely free at the front but supported 

 at the rear by an abbreviated septum. 



Dorsal interior. — Notothyrial cavity small and shal- 

 low; brachiophores long and slender, supporting plates 

 shallow, meeting the median septum. Cardinal process 

 a linear ridge not continuous with the median ridge. 

 Outside the brachiophores are concave plates ; between 

 these and the wall of the valve is located the socket 

 which is formed by a fulcral plate. Adductor scars 

 were not observed. 



Geologic range. — Lower Devonian of eastern 

 North America and ? Russia. 



American Species 

 Skenidium insigne (Hail) 1859 



Foreign Species 

 ? S. urdicum Tschernyschew 1887 



Distinguishing characters. — Skenidioides w.-js 

 separated from Skenidium chiefly because it does not 

 possess the peculiar hinge-plate characteristic of the 

 dorsal valve of the latter. There are also minor differ- 

 ences in the ventral valve. For example, in Skenidium 

 the spondylium at the posterior extremity of the valve 

 fills the delthyrium and is flush with the surface of the 

 palintrope. In Skenidioides, on the other hand, this 

 plate is always depressed below the level of the inter- 

 area. The spondylium of Skenidium in its growth 

 forward, however, becomes depressed below the inter- 

 area and its free end is attached to the outside of the 

 ridges formed during the progressive growth of the 

 teeth. 



In the dorsal valve the development of a flat or 

 flatly concave plate between the outside margins of the 

 brachiophores and the walls of the valve produces the 

 prominent hinge-plate characteristic of the genus. The 

 brachiophore supports are shallow and their inner mar- 



gins are upturned where they unite with the median 

 ridge. The cardinal process appears to be formed of 

 their united upturned edges. The cardinal process 

 and median septum are not therefore continuous. The 

 median septum divides the valve into two halves as in 

 Skenidioides. 



Discussion. — Hall and Clarke maintained that 

 Skenidium evolved from Orthis tnerofe Billings 

 through S. halli (Safford) to S. fyramidalis (Hall). 

 This genetic evolution, however, is not so simple, since 

 Orthis merofe is preceded in time by S. halli from the 

 Stones River (Glade limestone) of Tennessee and 

 S. anthonensc (Sardeson) from the Black River of 

 Minnesota. To the more primitive forms such as 

 S. halli and S. anthonense the writers have given the 

 name Skenidioides. It is likely that Skenidium devel- 

 oped directly from Skenidioides. The Orthis mcrofe, 

 on the other hand, appears to belong to a stock related 

 to Hesperorthis rather than to Skenidium. 



Skenidium. and Mystrophora have usually been 

 treated as synonyms, but the endopunctatc shell and 

 lobate cardinal process of the latter indicate that it is 

 a development parallel to Skenidium out of some dal- 

 manelloid stock. Mystrophora is accordingly placed in 

 the Dalmanellacea. 



Family ORTHID.E Woodward 1852, 

 emended 



(Orthinse (partim) Waagen 1884) 



Progressive and terminal Orthacea derived probably 

 out of the Eoorthinae, with interareas on both valves; 

 nearly all have open delthyria, oval or obcordate ven- 

 tral muscle fields, subrrtiiform ovarian impressions, and 

 subparallel median pallial sinuses. In the dorsal valve 

 the cardinal process is a simple linear ridge directed 

 ventrally; the brachiophores are rodlike, divergent, 

 and without support other than the adventitious shell 

 substance of the notothyrial platform; chiiidia are of 

 rare occurrence. 



The family may have had its origin in the Middle 

 and certainly in the Upper Cambrian and appears to 

 have died out with the Silurian. 



The family Orthida; as here emended is rather 

 radically reorganized and greatly restricted. Some of 

 its previously familiar members such as Plectorthis, 

 Hebertella, and Platystrophia have been placed in the 

 new family Plectorthidas. Such changes as these have 

 been brought about by a closer analysis of the internal 

 structures of the shell than any hitherto made, and a 

 more precise definition of the cardinalia. 



We base family characters on the gross internal 

 morphology, and to differentiate the genera we use 

 the highly variable external form, plus smaller details 

 of the interior. 



