426 CAMBEIAN BRACHIOPODA. 



PALaiOBOLTJS Matthew.a subgenus of OBOLTTS. 



[jraiaids; aucient; and Obolus.] 



Palxobolus Matissw, 1899, Bull. Nat. Hist. Soc. New Brunswick, vol. 4, pt. 3, No. 18, pp. 201-202. (Described and 



discussed as a new subgenus, likely of Obolus, though that does not appear in the text.) 

 Palxobolus Mattsev , 1903, Geol. Survey Canada, Kept. Cambrian Rocks Cape Breton, pp. 140-141. (Copies the 



first paragraph of the preceding reference and mentions the form as a subgenus. On subsequent pages (144 



and 146) the name occurs as Obolus {Palsiobolus).) 

 Obolus (Palxobolus) (Matthew), Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 4, PL XI, and pp. 142 and 144. 



(Classification of subgenus.) 



Characterized by an approximation of the main vascular canals and surface markings. 

 The surface is of the same type as that of Oiolus (Acritis) antiquissimus (Eichwald) (PI. XIII), 

 but the position of the visceral cavity and vascular canals differs materially in the two forms. 



Type. — Palseoholus hretonensis Matthew. 



Obolus (Palxobolus) beetonensis (Matthew). 



Plate XXXII, figures 5, 5a-g. 



Palxobolus bretonensis Matthew, 1899, Bull. Nat. Hist. Soc. New Brunswick, vol. 4, pt. 3, No. 18, p. 202, PI. II, figs. 

 2a-i. (Described and discussed. The first reference, though it is not described as a new species.) 



Obolus bretonensis Matthew, 1902, Trans. Roy. Soc. Canada, 2d ser., vol. 8, sec. 4, No. 3, p. 95, PL I, figs. 5a-€. (Char- 

 acterized. Figs. 5a-e are copied from Matthew, 1899, PL II, figs. 2a-e.) 



Palxobolus bretonensis Matthew, 1903, Geol. Survey Canada, Rept. Cambrian Rocks Cape Breton, pp. 141-143, PL 

 IX, figs. 2a-h. (Original description, Matthew, 1899, p. 202, copied and species redescribed and discussed. 

 The species is mentioned on p. 143 as Obolus bretonensis. Figs. 2a-h of this reference are copied from Matthew, 

 1899, PL II, figs. 2a-e, 2i, 2g, and 2h. Some of the specimens used by Matthew in preparing his illustrations are 

 copied in PI. XXXII of this monograph. Close identification is impossible, but figs. 5, 5b, 5c, and 5e of PL 

 XXXII may represent the specimens used by Matthew for figs. 2b, 2d, 2a, and 2e, respectively, of PL IX of his 

 paper.) 



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

 sometimes transversely ovate, the cardinal slope meeting at the beak to form a very broad 

 angle. Valves moderately convex in all specimens examined. Surface of shell marked by 

 strong, sharp, elevated, concentric ridges that merge into each other more or less toward the 

 posterolateral edges of the shell, where they terminate on the margin (PL XXXII, fig. 5c); 

 the space between the ridges slopes rapidly down into the rounded hoUow and more gently up 

 the anterior slope, which gives the effect of a subimbricated siu-face to the shell; fine radiating 

 strise mark the space between the ridges, and some of the better-preserved ridges are marked 

 by faint depressed strise; the irregularity of the ridges is most marked where they inosculate, 

 especially toward the sides; the inner layers of the shell are marked by concentric lines of 

 growth and fine radiating striae, and the inner surface by both sets of strise and munerous strong 

 pits scattered over the surface and gathered like beads on the concentric growth lines. The 

 shell is formed of the usual thin outer layer and numerous inner layers or lamellae, but it is 

 usually compressed in the shaty, impure sandstone so as to destroy the details of its lamellated 

 character. The size and relative proportion of the valves are shown by the illustrations. 



Casts of the interior of the ventral valve show a well-defined area that extends weU out 

 on the cardinal slopes; it is divided midway by a strong pedicle groove and again by a clear 

 but not prominent flexure line; it is also marked by rather strong growth lines parallel to its 

 base. Casts of the area of the dorsal valve show a rather strong area marked by lines of growth. 

 The casts of the interior of the valves show (a) a trace of the visceral area of the ventral valve, 

 (b) the position of the posterior of the main vascular sinuses in the ventral valve, and (c) a sharp 

 median ridge and the bifurcation of the scars left by the vessels in front of the anterior lateral 

 muscle scars in the dorsal valve anterior to the parietal line. The only traces of the muscle 

 scars are in the dorsal valve, where the centrals and small anterior laterals are faintly shown 



o The synonymy for this subgenus includes only those references in whicti the subgenus is discussed or described. To complete the record 

 the following mere references are listed; 



Obolus Matthew [1902c, pp. 94 and 95]. Obolus (Palxobolus) Matthew [1903, pp. 144 and 146). 



