OBOLID^. 431 



Two species from the Lower Ordovician may be referred to Lingulobolus, namely, 

 0. (L.) ajfinis and 0. (L.) sinssus. 



The tliickening of the shell mentioned by Matthew is a common feature in Obolus, and I 

 do not find the arrangement of the muscle scars to vary materially from those of 0. apollinis. 



Type. — Lingulella ? ajfinis BilHngs. 



Obolus (Lingulobolus) affinis (Billings). 

 Plate XVI, figm'es 1, la-e. 



Lingulella ? affinis Billings, 1872, Canadian Naturalist, 2d ser., vol. 6, No. 4, p. 468; fig. 4, p. 467. (Described.) 



Lingulella ? affinis Billings, 1874, Geol. Sui-vey Canada, Paleozoic Fossils, vol. 2, pt. 1, p. 67, fig. 35, p. 66. (De- 

 scribed. Fig. 35 is copied from fig. 4 of the preceding reference.) 



Lingulella affinis Billings, 1882, Geol. Sui'vey Newfoundland, Kept. Progress for 1881, Appendix, pp. 15-16. (Copied 

 from preceding reference.) 



Lingulepis affinis (Billings), Waloott, 1889, Am. Jour. Sci., 3d ser., vol. 37, p. 381. (Mentioned under new generic 

 name.) 



Linguloholus affinis (Billings), Matthew, 1895, Trans. Roy. Soc. Canada for 1895, 2d ser., vol. 1, sec. 4, No. 13, pp. 

 261-262, PI. I, figs. 4a-b. (Original description, Billings, 1872b, p. 468, copied and species described and 

 discussed.) 



Linguloholus affinis cuneata Matthew, 1895, idem, p. 262, PI. I, figs. 4c and 4d. (Described and discussed. This is 

 the first reference to this variety.) 



Obolus (Lingulobolus) affinis (Billings), Waloott, 1898, Am. Jour. Sci., 4th ser., vol. 6, p. 327. (New locality men- 

 tioned.) 



Lingulobolus affinis (Billings), Geabatj, 1900, Occas. Papers Boston Soc. Nat. Hist., No. 4, vol. 1, pt. 3, pp. 621-622. 

 (Described.) 



General form subtriangular, with the ventral valve subacuminate and the dorsal valve 

 slightly rounded at the beak. There is a little variation in the outhne of the valves, some 

 being slightly longer in proportion to the width. The valves are strongly convex, the dorsal 

 being the most so. A dorsal valve 30 mm. in length has a convexity of 5 mm. ; a ventral valve 

 of the same length has a convexity of 4 mm. above the plane of the margin. 



The surface of the shell is marked by fine radiating striae and concentric hnes of growth, and 

 very fine, concentric, sKghtly undulating, imbricating striae; when the outer surface of the 

 shell is exfoHated the surface of the inner layers shows rounded, radiating striae crossed by the 

 concentric Unes of growth, and fine, sHghtly irregular, concentric striae. The interior surface 

 over the visceral cavity is mai'ked by minute pits or punctae that in the anterior portion of 

 the shell are arranged in concentric Hnes, giving a highly ornamental appearance to the shell 

 when examined by a strong lens. 



The shell is thick, being built up of a thin outer layer and nurherous inner layers or 

 lamellae that are obHque to the outer surface over the anterior and lateral portions of the shell. 

 There is a strong development of the inner lay'erg of the shell over the visceral area, the impres- 

 sions of the muscle scars and vascular markings showing on the various layers as though there 

 had been a rapid deposit of shell substance beneath those parts. Usually the direct point of 

 attachment of the muscle is deeply impressed in the shell substance, but not always so. 



The longest ventral valve in the collection has a length of 30 mm.; width, 25 mm. A 

 large dorsal valve has a length of 32 mm. and a width of 29 mm. 



The interior casts show that the area of the ventral valve is not very clearly defined; it 

 is rather long and divided midway by a cast of a rather narrow pedicle groove; it is very rarely 

 that any traces of the area can be observed. The area of the ventral valve is obscure in most 

 specimens; one example shows that it forms a shelf, the undercut projecting slightly beneath 

 it so as to form in the cast two rounded projections extending toward the beak, on each side 

 of the median Hne, giving very much the same appearance as the same parts in the dorsal valve 

 of Lingulella granvillensis (Walcott) (PI. XXII, fig. Id). 



The cast of the visceral cavity of the ventral valve is very much Uke that of Oholus matinalis 

 (Hall) and O&oZus {Westonia) rogersi (Walcott). The material is somewhat imperfect, but 

 there is an outhne of the heart-shaped cavity (x), and the position of the space including the 

 central, middle lateral, and outside lateral muscle scars is indicated (PI. XXII, j&gs. 1, lb). 



