BRACHIOPODA. 



339 



Z-. «■ 



Larger valve. Smaller valve. 



Figs. 38, 39. Diagrammatic view of Obolus Quenstedti. After Mickwitz. 

 a. Pedicle- groove; b, impression left by the advance of the' lateral sliding muscles; c, median septum; d, cordate pit; 

 e, sinus; /, median groove; g, median swelling; It, lateral grooves; i, impressions of vascular trunks; k, impressions of 

 secondary sinuses; I, impressions of posterior adductors; m, impression of anterior adductors; n, impressions of sliding 

 muscles; o, outline of splanchnocojle; I, splanchnocoele; II, braohiocoile; III, pleurocojle. 



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 structui'e of O. Quenstedti with the 

 genus Obolella, Billings, as shown in the figures on Plate II of this volume. 



