OBOLIDiE. 471 



other genera. The presence of a pecuHar central channel in the cardinal area was the only 

 character of importance observed by Davidson [1866, p. 55] that served to distinguish Lin- 

 gulella from Lingula. In the original description, Salter [1866b, p. 333] calls attention to the 

 resemblance of the muscular scars of Lingulella to those of Obolus, but he considers that the 

 difference in relative position is sufficient to distinguish the two genera. Attention is also called 

 [Salter, 1866b, p. 333] to Obolella Billings, and to the fact that tlie later figures of Billings "show 

 a very different set of muscular scars." Davidson had the same material tiiat Salter had and 

 more, but was unable to find any satisfactory interiors, and hence left the genus as doubtful, 

 evidently considering it as closely related to Lingula. 



Mckwitz met with the same difficulty as Davidson, in having unsatisfactory material upon 

 which to base an opinion. After stating [1896, p. 126] that Ololella Billings would probably 

 have to make room for the genus Oholus Eichwald, he says: "Whether Lingulella Salter will 

 share the same fate I will not venture to predict with the same degree of certainty, since the 

 diagnosis and figures are even more imperfect than in Billings's genus." 



Wlien in Wales, in 1888, I made a small collection of Lingulella davisi (McCoy) at thetype 

 locality at Portmadoc, and later Mr. G. J. Williams sent me a number of fine specimens for 

 study. With this material, supplemented by a beautiful series of specimens of a number of 

 American species, it has been possible to obtam a clear conception of Lingulella and its relations 

 to Obolus, Oholella, Schmidtia, Lingulobolus , Lingulepis, and Lingula. 



The memoir of Mckwitz [1896] gave the genus Obolus a position that it had not held prior to 

 his very thorough investigation. With Ms descriptions and plates and a fine suite of specimens 

 worked out from material very generously given me by Dr. Fr. Schmidt, I have been able to 

 make a series of comparisons with Lingulella that at times has led me almost to doubt the 

 advisabiHty of characterizing Lingulella as generically distinct from Obolus. Tliis distinction is 

 now based on the more elongate form of most of the species of Lingulella, and the greater tliick- 

 ness of the shell of the typical forms of Obolus. There are differences in the position, size, and 

 form of the muscular scars, visceral area, and vascular canals of Obolus and Lingulella, but they 

 are not greater than those between different species referred to Lingulella. The same general 

 arrangement of muscle scars prevails, but on comparing the interior of the dorsal valve of 

 Lingulella davisi (McCoy) (PL XXXI, fig. 6e) with that of Lingulella acutangula (Roemer) 

 (PI. XVII, fig. Ih) or Lingulella ampla (Owen) (PL XXVIII, fig. Ih) we find as great variation 

 as when the comparison is made with the dorsal valve of Obolus (PL VII, figs. 6-9). The same 

 is true of the ventral valve, although the means of comparison are in this case not so good. 

 One of the oldest species of Lingulella, L. granvillensis Walcott (PL XXII) of the upper 

 Olenellus zone, has the outline of Obolus {Sclimidtia) celatus Volborth, and the interior markings 

 of the ventral valve are also of the same type. Lingulella acutangula (Roemer) (PL XVII, 

 fig. If) has the heart-shaped pit so characteristic of Obolus (PL VII, figs. 1-4) and the arrange- 

 ment of the muscular scars is essentially as in Obolus, but the outline of the valve is much more 

 elongate. Lingulella davisi (McCoy) and Lingulella ampla (Owen) vary decidedlj^ from Obolus 

 in the interior markings, but not more than Lingulella ampla differs from Lingulella acutangula. 

 The variations are so well shown by the figures on the plates that detailed comparisons will 

 not be entered upon. 



Matthew [1899b, p. 201] proposed to distinguish Obolus from Lingulella by its — 



roundness of outline, short cardinal area and depressed beaks, advanced position of muscle scars in the valve, and 

 strong arch of the vascular trunks in the ventral as well as the dorsal valve. There is, however, a more important 

 distinction, which, in consequence of imperfect preservation of the valves, can seldom be observed ; that is, the position 

 of the secondary muscles of the central group in the ventral valve, as compared with the great muscle of that group. 

 In Oholus they are lateral, but in Lingulella anterior to the great muscle. This shows a radical difference of structure 

 between the two genera. 



I am not able to recognize as constant the characters mentioned by Matthew, owing to 

 the great variation in the large group of species referred to Obolus and Lingulella. The data 

 for a clear distinction of the two, Obolus and Lingulella, are still too limited for more tlian an 

 arbitrary reference of Lingulella to a generic place in relation to Obolus. This is particularly 



