150 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



set appears at the line separating the anterior and pos- 

 terior adductor sets, each trunk bifurcating into sub- 

 sidary ones (see discussion below). 



Geologic range. — Late Silurian (upper Ludlow) 

 to Middle Devonian. 



American Species 



Dalmanella arcuaria Hall and Clarke 1 892 

 D.ferelegans (HaU) 1857 

 D. fygmtta Dunbar 1920 

 D.rockhousensis Dunbar 1920 

 Schizofhoria frofinqua (HaU) 1847 



European Species 



Dalmanella (/.) szajnochai Kozlowski 1929 



Ortkis canalicula SchnuT 185 3 (1851?) 



O. decifiens Barrande 1 847 



O. loveni Lindstrom 



O. neglecia Barrande 1 847 



O. occlust Barrande 1847 



O. tetragona Roemer 1844 



O. irigeri Verneuil 



Distinguishing characters. — Isorthis is distin- 

 guished from Dalmanella by its strongly biconvex 

 lateral profile, lack of prominent fold and sulcus, pecu- 

 liar ventral and dorsal musculature, and brachiophores 

 with fulcral plates. 



Discussion. — The ventral musculature with its 

 elongate and divergent diductor impressions most 

 strongly resembles that of certain species of Schizo- 

 fhoria. The diductor tracks are deep and elongate and 

 are separated by a prominent ridge which carries the 

 adductor muscles. A strong pallial trunk extends an- 

 teriorly from the ends of the diductor impressions. 

 Branching of these primary trunks has not been 

 observed. 



In the dorsal valve the cardinal process is small, but, 

 contrary to Kozlowski's statement, it has a lobate myo- 

 phore, as shown by /. canalicula (Schnur) and other 

 species placed in the genus. The pallial sinuses are 

 well shown in /. tetragona^ in which four trunks appear 

 to originate at the antero-median extremities of the 

 anterior adductor scars. In reality there are only two 

 main trunks but they bifurcate almost at their origin 

 and send out two secondary branches, one pair antero- 

 medially, the other antero-laterally. A second pair of 

 primary pallial sinuses is given off from the horizontal 

 elevation dividing the adductor scars. This trunk 

 bifurcates in about half the distance to the margin, 

 sending one branch antero-laterally, the other postero- 

 laterally. In makeup these sinuses are much like those 

 of Schizophoria, as seen in S. tuUiensis and S. senecta, 

 in which all the trunks originate at the same place as in 

 Isorthis, but since the shell ridge dividing the adductor 

 scars is oblique they extend toward the front in a sub- 

 parallel arrangement. Schizofhoria frofinqua (Hall) 

 has pallial markings exactly like those of Isorthis and 

 is therefore provisionally placed in that genus. 



Of interest in Isorthis is the great development of 

 adventitious shell deposited upon the cardinalia and 

 in the vicinity of the dorsal muscular area. In some 

 instances primary structures are nearly completely 

 obscured. 



Isorthis appears to be common in Europe, being 

 well represented in the Lower Devonian of Bohemia 

 and in the younger Eifelian of Germany. In America 

 it is common in the Helderberg (New Scotland) 

 (/. ferelegans, I. rockhousensis, I. fygmcea), and in 

 the Silurian of Tennessee it is represented by /. arcuaria. 



Family LINOPORELLID^ Schuchert and 

 Cooper 1931 



Aberrant, specialized Dalmanellacea, having a dor- 

 sal cruralium and a shell ornamentation recalling 

 Porambonites, 



Geologic range. — Silurian, with the genera 

 Linoforella Schuchert and Cooper and Orthotrofia 

 Hall and Clarke. 



Discussion. — The ventral musculature of Linofor- 

 ella is close to that of Pionodema and Schizofhoria. 

 In the dorsal valve, however, the convergence of the 

 brachiophore plates to meet a median septum is such 

 a radical departure from the usual structure of the 

 Schizophoriidae as to warrant the erection of a family 

 to recognize this variation. The schizophorioid ventral 

 muscles and the punctate shells indicate the probable 

 origin of Linoforella, but the presence of a cruralium 

 shows a distinct divergence from some stock as yet 

 unknown, but probably to be looked for among the late 

 Ordovician Schizophoriidae. 



Genus LINOFORELLA Schuchert and Cooper 

 1931 



PI. 18, figs. 13, 14, 17, 18,24,33 



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

 p. 247. 



Genoholotype. — Orthis functata Verneuil 1848, 

 Bull. Soc. Geol. France (2), vol. 5, p. 343. 



Description. Exterior. — Semioval to subglobose, 

 margins convex; hinge-line narrower than the greatest 

 width of the shell; cardinal extremities rounded; 

 lateral profile subequally biconvex; anterior commis- 

 sure slightly sulcate; ventral interarea the longer, 

 curved, apsacline, beak strongly incurved, delthyrium 

 open; dorsal interarea anacline, beak incurved; sur- 

 face multicostellate, the striae being marked by a row 

 of coarse pits which do not perforate the interior shell 

 layer. Test fibrous, punctate. 



Ventral interior. — Delthyrial cavity deep ; teeth 

 small, fossettes lacking; dental plates strong and 



