330 GENERAL SUMMARY TO 



sand by the sliding motion of the two valves, using at the same time the fringes of setae, 

 which swung promptly back and forward, like a bank of oars, leaving a peculiar track in 

 the sand. In the motion of the setae he noticed the impulse commencing from behind 

 and running forward. 1 



Muscular scars are beautifully displayed on the interior surface of the valves of 

 many fossil forms, and they can be understood now that their functions have been deter- 

 mined from their study in the recent species. They vary, however, a great deal in shape 

 and position in some genera. 



It was some time before Prof. King and myself could interpret the characters of the 

 scars, and the assumable functions of their muscles, on the inner surface of the valves in 

 the Trimerellida. After much research we arrived at the conclusion that this group 

 was most nearly allied, though differing in many respects, to that division of the 

 Brachiopoda of which Lingula may be considered as the type. The differences between 

 the two groups, we contended, were quite sufficient to warrant the step we took in making 

 of each a distinct family ; while on the other hand, the points of agreement are so numerous 

 as to show that they are intimately related. Obolus is placed by Dall in the family 

 Lingulida ; but its close relationship to Distinct opposes the allocation ; this alone 

 prevents our placing the Trimerellids in any family group which embraces the Obolus, 

 though we are quite ready to admit that both belong to one and the same great division 

 in their class. 



The muscles in Crania and the impressions they left in the interior of the valves 

 have been described and illustrated in page 192 of my * Monograph of Carboniferous 

 Brachiopoda,' Vol. II. Those of Discina were described by Sir R. Owen in his memoir 

 on the "Anatomy of the Brachiopoda" in the 'Transactions of the Zoological Society,' 

 vol. 1, p. 153, November, 1833, and at page 127 of my General Introduction in Vol. I 

 of this Monograph. We do not yet, however, possess satisfactory anatomies of the two 

 genera above alluded to. 



The brachial appendages (Vol. V, PI. XXI, figs. 1, 3, 4) are a pair of very remarkable 

 and beautiful organs eminently characteristic of the Brachiopoda. They are often, and 

 more correctly, termed labial appendages on account of each member being a prolonga- 

 tion of the lateral portions of the lips or margin of the mouth. Spirally fringed labial 

 appendages were first noticed by Pallas in Terebratula and by Otto F. Miiller in Crania. 



The labial appendages assume different shapes in different genera, and are supported 

 or otherwise, by the more or less complicated calcified lamellae already described. 

 The labial appendages, whether they form a pair of long, bent, or spirally convoluted 

 organs, occupy the larger portion of the cavity of the shell in front of the visceral 

 chamber ; they therefore are mainly composed of a membranous tube fringed on one 

 side with long flexible hollow cirri which were not capable of being protruded in those 



1 " The Brachiopoda, a Division of Annelida," 'American Journal of Science and Arts,' vol. i, 1870. 



