NO. 1229. CAMBRIAN BRACIIIOPODA—WALCOTT. 681 



to the LingulicUv. and that there is no good reason for cstablishino- the 

 famil}' 01)olida\ 



Their points of simihirity as described by Micl\witz are: 



1. The chemical constitution and microscopic structure arc the same. 



2. The position of the umbonal muscle is the same in the dorsal 

 valve. 



3. The arrangement of the vessels of the circulator}' system is the 

 same in the two genera. Mickwitz sa3's of this:^ 



Issuing between the same muscle sears from the splanchnoccele, two main vessels 

 extend in each valve into the fore part of the mantle loljes and brani'h 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 Obolu.s this takes place only in the large 

 valve. In the small valve the main ves.sels, shortly before reaching the perij^heral 

 canals, l)end into the interior of tlie valves and end at the scars of the anterior lat- 

 eral muscles. 



■i. The general arrangement of the muscle scars is essentially the 

 same, the points of difference being of a generic character.^ 



Their points of difference, according to Mickwitz. are: 



1. In the area of the valves. 



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

 umbonal muscle of the ventral valve, while in Obolus it is situated 

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



3. Quoting from ]\Iickwitz: 



Besides the somewhat unlike arrangement of some scars, to which we shall pre- 

 sently return, the bipartition of certain muscles constitutes the most characteristic 

 difference in the internal organization of the two genera. The umbcjnal muscle of 

 Obolus, which is divided on the side of the large valve, while conversely the two 

 transmedial muscles of Lingula, one of which is divided throughout its length, are 

 represented in Obolus by a pair of undivided muscles. 



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 jiedicle 

 muscle. On the contrary, the transmedial muscles, besides their bipartition, pre.sent 

 other differences. In Obolus the scars of that part of muscles on the large valve are 

 combnied with those of the anterior lateral muscles, while in the corresponding shell 

 of Lingula, though lying in a similar position, they are separated from the anterior 

 lateral muscles. With the small valves the case is reversed. 01)olus shows the scars 

 of the pair of muscles in question isolated, while in Lingula they are united with 

 those of the middle and outside lateral muscles. 



The scars of the two last-named muscles on the small valve of Oljolus 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 transmedial 

 muscle in Lingula) consists in the reversal of their combination. In Obolus, on the 

 large valve, the scars ( and j are united, in Lingula they are separated ; in Lingula, on 

 the small valve, i and /;, /, are uniteil, while in Obolus they are separated. 



The scars of the anterior lateral nuiscles of the small 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, while in Lingula they are nearer to the center 

 of the valve and are' united. 



^UL'er die Brachiojiodengattung Obolus Eichwald, \i. 121. 



