72 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Ventral interior. — ^Teeth strong, located at the angle 

 formed by the delthyrial and hinge margins. Dental 

 lamella? forming a shallow spondylium simplex, sup- 

 ported at the rear of the shell but free at the front. 

 Muscle-scars borne on the spondylium. 



Dorsal interior. — Notothyrial cavity shallow, brach- 

 iophores long and slender, supporting plates converg- 

 ing and uniting with the median septum to form a 

 cruralium. Cardinal process linear, prolonged toward 

 the front as a prominent median ridge; myophore a 

 compressed crenulated area. Sockets defined by a 

 concave socket plate. 



Geologic range. — Middle Ordovician (Stones 

 River) to Silurian (Niagaran: Wenlock and Lock- 

 port). 



American Species 



Skenidioides billingsi Schuchert and Cooper 1931 



Skenidium anthonense Sardeson 1 892 



S. halli Sa£Ford 1 869 



S. -pyramiddis (Hall) 1852 



?? O. merofe Billings (may be a Hesferorthis) 



European Species 



Skenidium craigensis Reed 1917 

 S. greenoughi Reed 1917 

 Orthis lezoisii Davidson 1848 



Distinguishing characters. — This group of tiny 

 shells has been usually identified with Skenidium and 

 externally its members do form a rather precise homce- 

 omorph with those of that genus. Internally, however, 

 Skenidioides differs from Skenidium in not possessing 

 the marked hinge-plate so characteristic of the latter. 

 Along with the decided development of the hinge-plate 

 in Skenidium has gone the loss of the dorsal interarea, 

 which is, however, well developed in Skenidioides. 



Discussion. — It is important to bring out in greater 

 detail here several points about the shell of Skenidioides, 

 leaving the detailed comparison of the two genera to 

 the discussion under Skenidium.. The spondylium of 

 Skenidioides is rather shallow and hangs freely into the 

 body chamber at its front end; at the posterior end it 

 is supported either by an abbreviated stout septum, or 

 by adventitious shell, forming a callosity beneath the 

 plate. The spondylium is attached to the ventral sur- 

 face of the palintrope along the outside of the ridge 

 formed by the progressive growth of the teeth. The 

 edges of the delthyrium are the inner margins of the 

 tooth ridges which project into the delthyrial cavity 

 when the whole structure is viewed from the front or 

 in section. 



In the dorsal valve the cardinalia are distinctive. 

 The brachiophore supports are discrete plates meeting 

 a high median septum. At the posterior of the valve 

 the cruralium rests on the inner surface of the valve 

 or on a callosity of adventitious shell deposited in the 



cavity between the plates and valve. The fulcral plates 

 are like those in Plectorthis and the socket may be cov- 

 ered over by the margin of the palintrope. The 

 median ridge and cardinal process form a continuous 

 partition which effectively divides the shell into a right 

 and left half. The adductor scars, according to Koz- 

 lowski's figure of Skenidium lewisi^' are located on 

 each side of the median septum. The anterior scars 

 are located postero-laterally of the other pair. 



The brachiophores and supporting plates are like 

 those of Plectorthis and lead to the inference that 

 Skenidioides is a branch from the early Plectorthis 

 stock, or better, from the earlier Finkelnburgia, as 

 Kozlowski suggests. Skenidioides developed along the 

 line of perfecting the spondylium and cruralium, but is, 

 nevertheless, wholly unrelated to any of the Clitam- 

 bonitidae or Pentameracea where it was previously 

 classified. Placement near the Plectorthidae is justi- 

 fied because of the complete lack of a deltidium or 

 chilidium and the plectorthoid character of the car- 

 dinalia. It has been shown under the Clitambonitidx 

 that the brachiophores characteristic of this family are 

 orthoid and that their support is accomplished, not by 

 plates, but by adventitious shell which swells laterally 

 from the notothyrial platform. In Skenidioides the 

 brachiophore plates form a decided cruralium, and in 

 Skenidium there is a distinct hinge-plate, both features 

 being foreign to the Clitambonitidas but clearly deriva- 

 ble from the Finkelnburgiidx or Plectorthidae. 



Skenidioides billingsi Schuchert and Cooper 1931 

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



Shell rather small, semicircular in outline, biconvex 

 in lateral profile. Ventral interarea long and broad, 

 strongly apsacline; delthyrium open. Dorsal valve 

 less convex than the ventral, with a rather short inter- 

 area. Surface marked by about 28 low costellas, a 

 few of which bifurcate beyond the middle of the shell. 

 The ventral valve is provided with a low median eleva- 

 tion and the dorsal valve is rather deeply sulcate. 



This shell is distinguished from S. anthonense 

 (Sardeson) by its larger size, coarser ornamentation, 

 and less proportionate width to its length. 



Measurements: 



Holotype 

 Paratype 



Length 

 5.5 mm. 

 4.5 



Width 

 8 mm. 



7.5 



Thickness 

 3.5 mm. 



S. billingsi comes from rocks of Black River age, 

 at Paquette Rapids, Ottawa River, Quebec. The 

 holotype is Cat. No. 2013a, Schuchert Collection, Yale 

 University. 



=== Pal. Polonica, vol. 1, 1929, p. 48. 



