40 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Orthotichia {Orthis (?) morganiana Derby). This writer, however, was mistaken as to the true 

 number in that genus. It is shown under Orthotichia, and also in our discussion of Schizofhoria, 

 that the posterior adductor scars migrate laterally, leaving a rather prominent space occupied by the 

 low median ridge only. It is this space that Derby took to be the scars of a third pair of muscles. 

 The diductor impressions of the dorsal valve are located either on the myophore of the cardinal 

 process or on the floor of the notothyrial cavity on either side of a linear cardinal process or septum 

 as in Hes'per orthis and Dolerorthis. In genera with a cardinal process of the Orthis type, it has been 

 possible to distinguish definitely only one pair of muscles, placed on either side of the process. 

 There is, however, a possibility that there were actually two pairs, one attached to the sides of the 

 process and the other to the notothyrial platform next to the process. In shells with the Dalmanella 

 type of cardinal process it is clear that there were two pairs of muscles attached to the myophore, as 

 shown clearly in the quadrilobation and trilobation of the latter. 



PALLIAL MARKINGS 



Our study of the Orthoidea shows that a definite pattern for the pallial sinuses of the dorsal 

 valve is fairly prevalent. In the early forms such as Finkelnburgia, Syntrophioides, Syntrophia, 

 Billingsella, and other related genera, the pallial markings in general have a radial arrangement, the 

 trunks generally taking their origin between the adductor impressions both on the sides and in the 



Figs. 1 1-15. — Dorsal pallial markings. 1 1, Orthis rotunda (Pander). 12, Schiozophoria. The usual condition in the 

 orthoid dorsal valve is a lateral trunk extending from the space between the anterior and posterior adductors, and two 

 trunks from the anterior end of the median ridge between the anterior adductor scars. These two trunks bifurcate immedi- 

 ately, giving rise to four. Cf. t. figs. 13, Isorthis; 14, Levenea; 15, Heterorthis clytie (Hall). 



front. In later forms the radial arrangement is lost but the trunks originate at essentially the same 

 point. In Schizophoria and Isorthis two trunks take their origin medianly between the left and right 

 adductor sets. These extend for a short distance and then bifurcate at or near the front of the 

 anterior adductor scars. The four resulting branches may extend directly anteriorly as in some 

 species of Schizophoria (t. fig. 12), or may run obliquely as is more usual. In addition to these 

 trunks and their subsidiary branches, a lateral trunk originates between the anterior and posterior 

 adductors, and extends directly laterad as in Isorthis and Levenea (t. figs. 13, 14). In some species 



