410 CAMBRIAN BRACHIOPODA. 



Monobolina rc/uZj7c?is'MATTHBW, 1903, Geol. Survey Canada, Kept, Cambrian Rocks Cape Breton, pp. 210-213, PL, ■ 

 XVI, figs. 2a-b, PI. XI, figs. 4a-b. (Copies the original description, Matthew, 1892, pp. 44-45, and describes 

 and discusses species. PI. XI, figs. 4a-b, are copied from Matthew, 1892, PI. XII, figs. 6d (ventral) and 6d 

 (dorsal); PI. XVI, figs. 2a-b, are copied from Matthew, 1902b, plate opposite p. 112, figs, lla-b.) 



General form transversely ovate, the dorsal valve being slightly more rounded at the beak 

 tlian the ventral. Valves rather strongly convex, as>seen in a single specimen of an imcom- 

 pressed dorsal valve; most of the shells are flattened in the shale. Sin-face of shell marked by 

 concentric lines of growth and very fine, irregidar strife that inosculate so as to produce a surface 

 much resembhng that of Oholus fragilis (W edcott) , Lingulella hellula (Walcott) , O&oZiis (Westonia) 

 ella (Hall and Whitfield), and other forms. The shell is not preserved on any of the specimens 

 in the collection. A ventral valve 6.25 millimeters in length has a width of 6.5 millimeters; a 

 larger dorsal valve 8 mm. in length has a width of 9 mm. 



The cast of the interior of the valves shows a clearly defined rather strong area on each 

 valve. The only muscle scars clearly defined are the central scars of the dorsal valve. 



Observations. — This species has the general form of Oholus mickvntzi Walcott (PI. X) but 

 in the absence of good specimens detailed comparisons are of little value. 



Matthew [1902b, p. 98] includes Obolus refulgens in the subgenus Monobolina Salter [1866b, 

 p. 334] on account of the close approximation of the "anterior adductor scars" of the dorsal 

 valve. 



The illustrations by Davidson [1866, PI. IV, figs. 20-27] of Monobolina plumbea (Salter) 

 indicate a platform of the type of that found in Elkania (PI. LI, figs. 1, la, and 4b), and as 

 Monobolina ■plumbea is a Lower Ordovician species it may be that the latter is a descendant 

 of Elkania of the Cambrian. It is certainly not an Obolus. 



Formation and locality. — TTpper Cambrian: (308 [Matthew, 1892, p. 45]) Shales of Division CSc of Matthew 

 [1892, p. 43], at Navy Island, St. John Harbor, New Brunswick. 



(307 [Matthew, 1903, p. 213]) Shales of Division C3c of Matthew [1901a, p. 276], on McLeod Brook (=Barachois 

 River); and (372e [Matthew, 1903, p. 50]) shales on McMullins Brook, near McLeod Brook; both in Cape Breton, 

 Nova Scotia. 



Obolus ehea Walcott. 



I 



Plate IX, figures 1, la-c. 



Obolus rhea Walcott, 1898, Proc. U. S. Nat. Mus., vol. 21, pp. 387-388. (Described and discussed as below as a new 



species.) 



General form elongate ovate, with the ventral valve subacuminate, and the dorsal valve 

 elongate ovate in outline. Outer surface unknown, as all of the shells referred to this species 

 are more or less exfoliated; the surface of the inner layers shows numerous, rather broad, radi- 

 ating striae, and concentric lines of growth. The shell appears to have been formed of a thin 

 outer layer and several inner layers or lamellse arranged in the same manner as in Obolus 

 matinalis (Hall). A ventral valve 8 mm. in length has a width of 6 mm.; a shorter, broader 

 valve is 6.5 mm. long and 5.5 mm. wide. The two dorsal valves referred to this species are 

 larger than the ventral valves; one 9.25 mm. long has a width of 6.75 mm. and another 9.25 

 mm. long has a width of 7.25 mm. 



Casts of the interior of the ventral valve show a rather short area that is not clearly 

 defined in any of the specimens. The cast of the pedicle groove is narrow and merges into the 

 cast of the groove extending forward to the visceral area; the area is also marked by flexure 

 lines and transverse strise of growth. The area of the dorsal valve is short in the one specimen 

 showing it. The cast of the visceral cavity on a ventral valve is clearly defined by a rather 

 narrow ridge that is expanded anteriorly in what may represent the heart-shaped cavity (v) 

 (PI. IX, figs. 1, la). The parietal scar passes around in front of the visceral cavity and then 

 a little backward to the main vascular sinuses. No traces of a median septum have been 

 seen in either valve. 



The only traces of muscle scars observed are some irregular markings in the trapezoidal 

 area (c), in which the central, middle lateral, and outside lateral scars occur in the ventral 

 valve. 



