BRA CHIOPODA. 
339 
a, Pedicle-groove; 6, impression left by the advance of the'lateral sliding muscles; c, median septum; d, cordate pit; 
e, sinus; /, median groove; g, median swelling; h, lateral grooves; i, impressions of vascular trunks; 7c, impressions of 
secondary sinuses; l, impressions of posterior adductors; m, impression of anterior adductors; n, impressions of sliding 
muscles; o, outline of splanchnocoele; I, splancknoccele; II, brachiocoele; III, pleurocoelc. 
According to this diagnosis, the analogies of Obolus, with Lingula are at 
once striking, though there are important differences. In L. anatina there are 
four pairs of lateral or sliding muscles, while there appear to be but three in 
Obolus Quenstedti ; according to Mickwitz the anologue of k (middle lateral in 
Lingula, see figure on page 10), being absent in the latter. The adductors are 
the same in number in both, though there is considerable difference in their 
disposition; the position of the posterior band, which is divided at its ventral 
insertion being the same as that of the great umbonal in Lingula. In Lingula, 
again, King has shown that one of the transmedian bands is divided, which 
does not appear to be true of Obolus Quenstedti. 
It must be observed that these results have not been obtained from the type- 
species of the genus, Obolus Apollinis, von Eichwald, but from a hitherto unde¬ 
scribed form. Experience has taught us that the greatest care is required in 
the discrimination of generic values among the oboloid genera of the early 
palaeozoic faunas, and it may be a question for subsequent determination whether 
a species showing so many important differences from the structure hitherto 
known in 0. Apollinis should be regarded as congeneric with it. Attention is 
called to the similarity in many points of structure of O. Quenstedti with the 
genus Obolella, Billings, as shown in the figures on Plate II of this volume. 
