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J. H, ORTON. 



practically subdivided into right and left portions by a median septum, 

 and that in others, as in Terebratula janitor, the shell is actually divided 

 into two wings (see Fig. 8 A and B, below). In all Brachiopods there would 

 thus appear to be little doubt of the physiological independence of the right 

 and left halves of the mantle cavity. Hence variations in the direction 

 of formation of antero-posterior median septa, as shown in Stringo- 

 cephalus, Conchidium, and many other genera, of trilobation of the shell, 

 as shown in many Rhynchonellidse, and bilobation of the shell, as shown 

 in Orthis biloba, Terebratula diphya, and other forms, do not interfere 

 with the functions of the two portions of the lophophore, and it is con- 



A B G 



Fia. 8. — Views of types of Brachiopod shells showing morphological division of the 

 mantle cavity into right and left portions, and trilobation of the shell. 



A. Shell of Terebratula (Pi/gope) janitor (after Oehlcrt) in which the mantle 



cavity is almost entirely divided into two separate compartments. 



B. View of interior of shell of Conchidium Knighti (after Davidson) showing 



internal partial subdivision of the mantle cavity by the septa which are 

 shaded. 



C. Frontal view of the shell of Bhynclioyiella cynocephala (after Oehlert), showing 



the anterior median portion of the shell produced into a sort of siphon 

 in the region in which in other Brachiopods (for example, Crania and 

 Lingula) is expelled the outgoing current. 



ceivable that these variations may be advantageous to certain forms 

 under certain conditions. 



The disposition of the lophophore in the Spiriferidse indicates that the 

 main ingoing current entered the mantle cavity in the front middle 

 portion and was expelled in two outgoing currents at the postero-lateral 

 angles of the shell, a condition exactly the reverse of that obtaining in 

 Crania, where the ingoing current is twofold and the outgoing current 

 single. Thus the production of the shell — frequently occurring in this 

 group — into postero-lateral angles, somewhat like those occurring in 

 modern Pectens, may have served as a sort of siphon for carrying away 

 the exhalent streams. 



