OBOLID^. 377 



Variation in the position of the vascular sinus of Obolus and its subgenera and Lingulella — Continued. 



Subgenera of Obolus. — -Acritis Volborth [1869, p. 212] is ornamented with concentric lines 

 which are elevated, irregular, and undulating. The valves are strongly arched and massive; 

 the visceral area (splanchnocoele) small and short, and the pedicle furrow conical and deeply- 

 impressed in the carduial area. Type: Obolus (Lucina) antiquissimus Eichwald (PI. XIII, fig. 

 3, and PL XV, figs. 1, la-c). 



Broggeria Walcott [1902, p. 605] has the typical ovate outline of Obolus, but difters from 

 Obolus and its other subgenera in having a very deep visceral depression (splanchnocoele) in 

 both valves, and in its minutely papillose interior surface. Type: Obolella salteri Holl (PL 

 XIII, figs. 1, la-n, and PL XV, figs. 4, 4a-d). 



Eoobolus Matthew [1903, p. 135] was proposed for certain species of Obolus in which the 

 central muscle scars and anterior scars were arranged in a more or less quadrate manner, as in 

 Obolus selwyni (Matthew) (PL XXXVI, figs. 2a and 2b). I find that the specimen illustrated by 

 Matthew [1903, PL VIII, fig. 2d] was distorted. It is figured in Plate XXXVI of this monograph 

 (fig. 2a), and a second distorted specimen in figure 2b. A more perfect specimen is illustrated 

 in figure Ig, and the changes in position of the muscle scars in the center of the dorsal valve 

 may be seen by comparing Plate XXXVI, figures 2a and Ig; Plate XXXVII, figure Ic; Plate 

 XXVIII, figures Ih and Ig; Plate XXXI, figure 6e; Plate XVII, figures Ih and Ij; Plate XXI, 

 figm-e 2a. A study of the figures cited and many others shows that the position of the central 

 muscle scars and anterior lateral scars is not a character of sufficient importance and value 

 upon wliich to found a subgenus of Obolus. On this accoimt Eoobolus is considered as a synonym 

 of Obolus. 



Euobolus Mickwitz [1896, p. 314] was elaborately described as a subgenus of Obolus by 

 Mickwitz, who referred to it all species showing radial'striation. He says [1896, p. 128]: 



In grouping the species by their distinctive marks, two great divisions become apparent. One of them shows, 

 besides the concentric striation of the outer surface of the shell, peculiar to all Obolus species, also a radial striation 

 which is lacking in the others. Thus, while the concentric striation is a general characteristic, which, nevertheless, 

 by differences in development, furnishes useful specific features, the radial striation, due to a different condition of the 

 edge of the mantle (no matter whether this be due to the mantle bristles or to the circulatory vessels), constitutes a 

 group character. For this reason all radially striated Oboli, whose other characters also show a certain agreement in 

 other respects, were united into a subgenus Euobolus. Furthermore, the concentric striae of certain species showed 

 remarkable variations, which also indicate a peculiar organization of the edge of the mantle. The species 0. silurieus 

 and antiquissimus show this striation running parallel on the growth lamellae corresponding to a period of vegetation, 



