372 CAMBKIAN BRACHIOPODA. 



in both striated parallel to the base, and traversed, from the point of the beak on, by the more or 

 less diverging lines of folding of the areal lamellae and a strongly marked pedicle furrow. 



Visceral area (splanchnocoele) on the ventral valve restricted to the back part, in the dorsal 

 valve drawn far forward into the vascular area (brachiocoele) and strongly indented laterally; 

 in both valves it extends backward up close to the base of the visceral (splanchnocoelic) part of 

 the area, which is bounded by the flexure line (folding of the area lamellae). The edge of the 

 mantle of the beak part is restricted to the side areas (pleurocceles), appearing between the 

 flexure lines of the area lamellae. 



Anterior part of the thickened posterior half of valve slopes down to the middle of the 

 valve, forming in the ventral valve a line concave to the beak, in the dorsal valve a sinus. The 

 lateral parts of the thickening are drawn out into thin side edges. In front of the base of the area 

 is a small median septum, which in the dorsal valve for the most part is less prominent, but in both 

 is often hardly perceptible by reflected light as an almost invisible crest. Beginning at the 

 median septum, there are in each valve two gradually deepening grooves for the main vascular 

 canals, which intersect the thickened part parallel to the edge of the shell. In the dorsal valve 

 these grooves combine with the sinus from the calcareous ridge in forming two projections 

 pushing themselves like horns into the middle of the valve. The traces of the main vessels of 

 the mantle lobes in the continuation of the visceral (splanchnocoelic) vascular grooves are 

 subparallel and project into the vascular (brachiocoelic) parts of the valves, in the ventral 

 valve terminating with their anterior ends in the peripheral vascular canal, in the dorsal valve, 

 shortly before reaching the vascular canal, bending about into the interior of the shell, and 

 vanishing at the scars of the anterior lateral muscles. The peripheral vascular canals in both 

 valves run along their edge from one side area (pleurocoele) to the other. Traces of secondary 

 vessels radiate in great number from the grooves of the main vessels toward the peripheral 

 canals and into the middle of the shell. 



In the median line of the ventral valve, between the median septum and the anterior edge 

 of the thickened part of the shell, there is a deep, heart-shaped pit, with its point directed 

 forward and having a shallow median groove. In the cavity of the dorsal valve there is a more 

 or less pronounced median ridge, extending into the anterior part of the valve and divided 

 longitudinally by a shallow median furrow. 



On each valve there are six pairs of muscular scars, and in the ventral valve in front of 

 the pedicle furrow there is the unpaired scar of the- pedicle muscle. The umbonal scars are 

 close to the base of the area, in the ventral valve divided, inclosing the scar of the pedicle 

 muscle between them, in the dorsal valve confluent in the median line of the valve. The scars 

 of the central pair of muscles are placed about centrally in both valves; in the dorsal one they 

 are isolated on the inner sides of the ends of the hornlike projections; in the ventral valve they 

 are united with the scars of the outside and middle lateral muscles, on both sides of the anterior 

 part of the heart-shaped pit. The transmedian muscles are undivided; their scars in both 

 valves are close to the base of the area in the line of prolongation of the flexure lines. The 

 anterior lateral muscle scars of the ventral valve adjoin the transmedians, overlapping them a 

 little on the outside; in the dorsal valve they are isolated, pushed far forward toward the frontal 

 margin and situated at the anterior end of the median crest separating them. The scars of the 

 second pair, the middle lateral muscles, in the ventral valve are combined with those of the 

 central and outside lateral muscles at the point of the heart-shaped pit; in the dorsal valve 

 they blend with those of the outside lateral muscles and lie against them on the inner sides of 

 their anterior ends. Finally the scars of the third pair, the outside lateral muscles, in the 

 ventral valve are combined with those of the central and middle lateral muscles and lie on the 

 outer sides of the former, while in the dorsal valve they are united with those of the middle 

 lateral muscles and lie obliquely outward before the scars of the transmedian muscles. In both 

 valves the parietal band closely circumscribes all muscle scars externally, crossing the furrows 

 of the main vessels and entering the visceral (splanchnocoelic) part of the area at its base on a 

 line with the flexure lines. 



The study of a series of the interior of the ventral valve of Oholus apoUinis (PI. VII) shows 

 that the arrangement of the point of attachment of the middle (k) and outside (I) laterals and 



