32 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



(4) Accessory diductor muscles. — In Laqueus and other terebratulid genera there is an 

 accessory diductor set located immediately posterior to the adductor scars. From the presence of 

 such marks in these recent shells it might be suspected that homologous muscle patches should 

 occupy a similar position in the orthids. Such scars have been described by Sardeson^' in Dalman- 

 ella emacerata, in which they are located just posterior to the adductors and between them and the 

 pedicle callist, hence in precisely the same position as in Laqueus. In shells like Dinorthis sub- 

 quadrata and Rhifidomella there may be a corresponding set of muscles associated with the adduc- 

 tors, although these were not clearly seen by us. 



According to our interpretation of this set of muscles as adjustors, there can be no correspond- 

 ing set of scars on the dorsal valve, since these muscles are here attached to the pedicle. 



PALLIAL AND GENITAL MARKINGS 



The mantle (or certain of its layers) is folded upon itself at various points, enclosing cavities, 

 or fallial sinuses, which contain the body fluids and frequently portions of the genital organs. 



Pallial markings. — The markings of the main pallial trunks are often seen in the Cambrian 



Figs. 3-5. — Ventral pallial and ovarian markings. 3, Orthis s. s.; this type characterizes the family Orthidae as here 

 described. 4, Schizoramma, in which the markings are essentially the same as in Orthis and Hesferorthis. 5, Dinorthis 

 (PUsiomys) subquairata (Hall) ; notice here that the main trunks extend from the anterior of the diductor scars. In the 

 ventral valve the pallial trunks in all orthoid brachiopods begin at the same place. 



genera of Atremata {Obolus, Lingulobolus, Lingulella, Bicia, Dicellomus) and of Neotremata {Obol- 

 ella, Acrothele, Botsfordia, Acrotreta). Accordingly these markings should be common among the 

 Cambrian Orthacea, and so they are. They are especially well seen in BilUngsella. 

 In living brachiopods, according to Thomson,^' 



The pallial sinuses take the form of great trunks, entering the mantle-lobes from the coelome, and generally 

 branching repeatedly towards the margins. The disposition of the sinuses varies considerably in different groups, 

 and it is probable that these characters have at least super family value in classification. 



The pallial marks have been found of great use in defining some of our families. 



In general, the pallial markings of the ventral valve are rather simple as regards the main 

 trunks. Their general arrangement is foreshadowed in the Cambrian genera of the Billingsellidas in 

 the two prominent trunks which radiate antero-laterally from the forward ends of the diductor scars, 

 thence splitting and diverging toward the front and sides of the valve. One of the anterior branches 

 of each trunk swings dorsally and runs nearly parallel to the lateral margin. The other, shorter 

 branches extend inward and may or may not unite near the mid-line of the valve (see pi. 1, fig. 10). 



^'Amer. GeoL, vol. 19, 1897, p. 93. 

 »«Brach. Morph., 1927, p. 11. 



