620 CAMBRIAN BRACHIOPODA. 



The largest ventral valve in the collection has a length of 14 mm. and a width of 13.5 mm. 

 A dorsal valve 13 mm. long has the same width. 



The area of the ventral valve slopes backward and rises from the plane of the margins 

 of the valve at an angle of about 45°. It is divided midway by a false pedicle area that 

 varies in width; it is sub triangular in outline, slightly depressed, and has a narrow median 

 furrow of varying depth that is marked by strong hnes of growth; on each side of the inner 

 end of this furrow the area is extended forward as a toothlike projection, very much as in 

 Bicia (PI. L) and Oholella (PL LV). The area is marked by transverse Unes of growth sub- 

 parallel to its anterior margin. The cast of the area (PI. LXXXIII, figs. Ic, Id, Ig, li, and 

 Ik) shows the pits made by the projections of the area next to the false pedicle furrow; it 

 was these pits in the cast of the ventral valve of T. insignis (PI. LXXXIV, fig. 5) that Matthew 

 [1893a, p. 276] mistook for the dental sockets of the dorsal valve. 



The splanchnocoele of the ventral valve is large and clearly shown; the low, broad, rounded 

 median ridge is about one-half the length of the valve; it broadens out near the pedicle opening 

 and again toward the front; it is marked by a narrow median depression back of the pedicle 

 opening, and, in front of the pedicle opening, by a central, elongate swell surrounded by a 

 narrow furrow. The pedicle opening is clearly defined on nearly aU casts of the valve; it is 

 situated about two-fifths the distance from the area to the front end of the ridge on which 

 it occurs. On each side, a short distance back of the front end, an area is outlined that extends 

 obliquely backward to the main vascular sinus; this area may be depressed below the general 

 surface or raised above it; it corresponds to the area in the ventral valve of Oholus, in wliich 

 the central muscle scars and the middle and outside lateral scars occur. (Compare PL LXXXIII, 

 figs. Ic, Id, with PL VII, figs. 5 and 13; also with PL XIV, figs, la and 4a.) The transmedian 

 and anterior lateral muscle scars occur on a rounded ridge just outside of the main vascular 

 sinuses (PL LXXXIII, fig. Id). The umbonal muscle scars are oval in outline and situated 

 close to the posterior end of the central median ridge, and between the ridge and the trans- 

 median muscle scars. The impressions made by the main vascular canals are broad, strong, 

 and continued to the anterior third of the valve; they originate beside the median ridge close 

 to the area, and curve outward and then inward beyond the center of the valve. 



The area of the dorsal valve is narrow and divided by a central, slightly depressed, tri- 

 angular space; it is marked by transverse hnes of growth and appears to be on the same plane 

 as the margins of the valves. The splanclmocoele of the dorsal valve extends forward beyond 

 the center of the valve; it is divided by a narrow, median septum, and has an oval depression 

 in it a little back of the anterior lateral muscle scars; the central muscle scars are large, oval 

 in outline, and situated on the outer slope of the rounded ridge about the central depression; 

 the anterior lateral scars are small and placed on the rounded ridge in front of the central 

 depression and close to the median septum. A low, rounded ridge extends obliquely forward 

 into the valve from each side of the median septum; these probably mark the position of 

 vascular canals extending forward from the visceral area; they are shown in Plate LXXXIII, 

 figures 11 and In. The transmedian and anterior lateral muscle scars are located close to 

 the outer edge of the main vascular sinus, well back on the posterolateral part of the valve. 

 The main vascular sinuses start back of the visceral area near the median septum and curve 

 outward and then forward about halfway between the central muscle scars and the margin 

 at the center of the valve, and then a little inward before terminating near the front margin. 



Ohservations. — This fine shell was the type of Matthew's genus Protosijphon [1897a, p. 68]. 

 I find it to be congeneric with Trematobolus insignis Matthew, and rather closely related to 

 that species. This may be seen by comparing Plate LXXXIII, figures Ic, Id, with Plate 

 LXXXIV, figure 5. The pedicle furrow and aperture is much hke that of ScJiizambon tijpicalis 

 Walcott (PL LXXXIV, figs. 1, la) except that it is in a thick shell, wliile that of the latter 

 species is in a relatively thin shell. In 1898 Mrs. Walcott and I collected a large series of 

 specimens at the typical locahty, as I wished to illustrate fully tins the oldest of the Siphono- 

 tretidse of the Schizambon type. The shell occurs above the Olenellus zone, at about the same 

 general horizon as T. insignis and T. pristinus. 



