682 PROCEEDIXGS OF THE XATIOXAL MUSEUM. vol.xxiii. 



The other scars of the lateral muscles on the large valve of Obolus also are quite 

 analogous in their position to the corresponding scars in Lingula. True, in their case, 

 also small displacements and changes of form occur, but yet I am unable to attach to 

 these any special value. The two genera show the scars of the outside lateral mus- 

 cles combined with those of the central muscles, but we have seen that in some spe- 

 cies of Eichwald's genus (0. trkingularis, 0. pamhri, and some species of the sub- 

 genus Sehmidtia) , the first-named scars separate from those of the central muscles 

 and change their subtriangular form, drawn out backward, into a rounded form, more 

 like that in Lingula. The scars of the middle lateral muscles of the large valve, on 

 the contrary, are only in Eichwald's genus comljined with those of the central nms- 

 cles, while in Lingula they are separated. It is probable, however, that some species 

 of the above-named subgenus share this peculiarity with Lingula. 



Finally, the scars of the central muscles of the two genera differ merely by their 

 somewhat different form in the large valve, and by their somewhat different position 

 with relation to the axis of symmetry on the small valve. It was pointed out, how- 

 ever, in speaking of the central muscles of Obolus, that the backward-protracted points 

 of the su1)trapezoidal scars in the large valve of the typical species (as well as those 

 of the outside lateral muscles) are lacking in the species of the subgenus Sehmidtia, 

 so that even in regard to form there is an agreement with Lingula. In the small 

 valve of Obolus the elliptic scars of the central muscles are parallel to the major axis 

 of the valve or somewhat converging behind, while in Lingula they are strongly con- 

 vergent anteriorly. 



To the altered position of the muscle scars in Obolus corresponds the modified form 

 of the parietal band. The latter in both valves of Lingula is rhombic, but in the 

 smaller valve it is drawn farther forward than in the larger. In Obolus the parietal 

 band on the small valve extends still farther toward the frontal edge than in Lingula, 

 and in its posterior part is more markedly bent inward from both sides, producing a 

 characteristic unequally three-lobed figure. The parietal band of the large valve of 

 Obolus, on the contrary, is subelliptie inform, and rather approaches that of Lingula. 



More important than this difference in form of the splanchnocade is the dif- 

 ference in the form of the mantle lobes, which is manifest from the position of the 

 posterior part of the parietal band. In Lingula the parietal band is moved away 

 from the base of the area, and thus constitutes a narrow space between the two 

 pleurocoeles, which space is occupied by the mantle lobes that extend around the 

 entire beak part of the valves. These mantle lobes of the beak are in the small 

 valve also covered with mantle bristles, while the border of the mantle of the large 

 valve, in the splanchnoccele part of the area (deltidium King) is free from bristles. 

 In Obolus, on the contrary, the posterior part of the parietal band is close to the base 

 of the splanchnoccele part of the area, whose lamellte, as we have seen, are l)ent up 

 at right angles to the plane of the valve, and therefore could not have been deposited 

 by mantle lobes resting against the valves. Hence the mantle lobes of Obolus 

 extended only as far as the pleurocoeles, and were lacking, as well as the mantle 

 bristles, in the splanchnoccele part of the area of both valves. At that point there 

 was only the muscular wall of the body connecting the two valves, from which the 

 pedicle emerged.^ 



Oholm and Olxjldla. — Authors have compared Obolella with Obo- 

 lus, and Mickwitz thought that they might possibly be congeneric.^ 

 The narrow pedicle slit in the area of the ventral valve of Obolella 

 opening into a cylindro-conical chamber is so unlike the pedicle fur- 

 row^ of Obohis that a distinct generic reference is necessitated by its 



^ tjber die Brachiopodengattung Obolus Eichwald, pp. 118-121. - Idem, p. 129. 



