OBOLID^. 375 



The points of similarity between Oiolus and Lingula, as described by Mickwitz, are : 



1. Cliemical constitution and microscopic structure. 



2. Position of the umbonal muscle in the dorsal valve. 



3. Arrangement of the vessels of the cii'culatory system. Mickwitz [1896, p. 121] says: 



Issuing between the same muscle scars (j-l, h in the ventral valve; 1, i, k-h in the dorsal valve) from the splanch- 

 nocoele, two main vessels extend in each valve into the fore part of the mantle lobes and branch inward and outward 

 into numerous secondary vessels. The only difference in the arrangement of the vessels consists in this, that in Lingula 

 the main vessels of the two valves empty into the peripheral canal, while in Obolus this takes place only in the large 

 valve. In the dorsal valve the main vessels, shortly before reaching the peripheral canals, bend into the interior of 

 the valves and end at the scars of the anterior lateral muscles. 



4. General arrangement of the muscle scars. These are essentially the same, the points 

 of diilerence being of a generic character. 



The points of difference between the two genera are: 



1. The areas of the valves. 



2. Attachment of the pedicle muscle. In Lingula the muscle is attached back of the scar 

 of the umbonal muscle of the ventral valve, whereas in Oiolus it is situated between the divisions 

 of the scar of the umbonal muscle of the ventral valve. 



3. Bipartition of certain muscles. Mickwitz says [1896, pp. 118-121]: 



Besides the somewhat unlike arrangement of some scars, to which we shall presently return, the bipartition of 

 certain muscles constitutes the most characteristic difference in the internal organization of the two genera. The 

 umbonal muscle of Obolus, which is divided in the ventral valve [gg, fig. 34A], is undivided in Lingula [g, fig. 34C] 

 while conversely the two transmedian muscles of Lingula [ii, fig. 34C], one of which is divided throughout its length, 

 are represented in Obolus by a pair of undivided muscles [i, fig. 34A]. 



The position of the umbonal muscle is the same in the two genera; at most it is somewhat crowded away from 

 the base of the area in Lingula, because of the pedicle muscle. On the contrary, the transmedian muscles, besides 

 their bipartition, present other differences. In Obolus the scars of that pair of muscles on the ventral valve are com- 

 bined with those of the anterior lateral muscles [ij, fig. 34A], while in the corresponding shell of Lingula, though lying 

 in a similar position, they are separated from the anterior lateral muscles. With the dorsal valves the-case is reversed. 

 Obolus shows the scars of the pair of muscles in question isolated [i, fig. 34B], while in Lingula they are united with 

 those of the middle and outside lateral muscles [i, k, and 1, fig. 34D]. 



The scars of the two last-named muscles on the dorsal valve of Obolus [k and 1, fig. 34B] are combined in a manner 

 analogous to those of Lingula, so that the whole difference in the arrangement of the scars in question (aside from the 

 bipartition of one transmedian muscle in Lingula) consists in the reversal of their combination. In Obolus, on the ventral 

 valve, the scars i and j are united, in Lingula they are separated; in Lingula, on the dorsal valve, i k, and 1 are united, 

 while in Obolus they are separated. 



The scars of the anterior lateral muscles of the dorsal valve have a closely similar position in the two genera, 

 except that in Obolus they are moved farther forward and are separated by the median ridge (p), while in Lingula 

 they are nearer to the center of the valve and are united [j, fig. 34D]. 



The other scars of the lateral muscles on the ventral valve of Obolus also are quite analogous in their position to 

 the corresponding scars in Lingula [k, 1, fig. 34C]. 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 

 muscles [1, figs. 34A and 34C] combined with those of the central muscles \h, figs. 34A and 34C], but we have 

 seen that in some species of the genus (0. triangularis, 0. panderi, and some species of the subgenus Schmidtia) the 

 first-named scars are separate from those of the central muscles and change their subtriangular form, being drawn out 

 backward into a rounded form, more like that in Lingula. The scars of the middle lateral muscles (k) of the ventral 

 valve, on the contrary, are only in Eichwald's genus combined with those of the central muscles (h), 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 

 ventral valve and by their somewhat different position to the axis of symmetry on the dorsal valve. It was pointed 

 out, however, in speaking of the central muscles of Obolus that the backward-protracted points of the subtrapezoidal 

 scars in the ventral valve of the typical species (as well as those of the outside lateral muscles) are lacking in the species 

 of the subgenus Schmidtia, so that even in regard to form there is an agreement with Lingula. In the dorsal valve of 

 Obolus the elliptic scars of the central muscles are parallel to the major axis of the valve or somewhat converging behind 

 Pi, fig. 34B], while in Lingula they are strongly convergent anteriorly [h, fig. 34D]. 



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 dorsal valve it is drawn farther forward than in the ventral [fig. 34D]. 



