ANIMALS. 77 



in CameropJioria. I am only acquainted with the internal structure of Magas pumilus 

 through the beautiful figures given of it by Mr. Davidson ;i I can therefore only 

 throw out the suggestion that the anterior part of the apophysis of this shell may also 

 be a muscular fulcrum. 



One of the features which distinguish the Palliobranchs as a class from all other 

 molluscs, is their branchial system. Instead of possessing a respiratory apparatus, in 

 the shape of laminated gills, as in ordinary bivalves, both lobes of the mantle are 

 rendered subservient to respiration, by means of numerous veins and arteries variously 

 and minutely ramifying towards the pallial margin.^ All the recent forms hitherto 

 examined, have the mantle closely and strongly adhering to the inner surface of the 

 valves ; and, it is in consequence of this adhesion, that we occasionally meet with 

 palliobranchiate fossils displaying, in the most beautiful and instructive manner, 

 impressions of the vessels that conveyed the blood which circulated in the long-since 

 extinct molluscs that inhabited them. 



Having succeeded in obtaining several specimens exhibiting impressions of nearly 

 their entire vascular system, it has been deemed necessary to give some figures to 

 illustrate a character of the greatest importance in the class under consideration. 

 The figures representing casts of both valves of CameropJioria multiplicata (PI. VIII, 

 figs. 6, 7) instructively display the large veins (?) or returning vessels inclosing the 

 arteries {j), described by Professor Owen as prevailing in the recent forms.^ 



The specimens of Leptcena analoga represented under figures 6 and 7 in Plate XX, 

 show the impressions of the vascular system in a beautiful state of preservation ; but 

 owing to the veins being obscured and confused near the anterior angles of the valves, 

 it is impossible to say whether the main trunks passing along their medio-longitudinal 

 region curve continuously round towards the centre of the hinge, or become confluent, 

 where the obscurity prevails, with those which pass along, and forward from, the 

 hinge-line, supposing the latter to originate in the medio-cardinal region. From what 

 is displayed on the figures of CameropJioria multijjUcata, the former view seems to be 

 the correct one. The cast of Trigonotreta undulata, under figure 6 of Plate IX (c), is 

 the only specimen of the genus I have seen exhibiting the vascular system. 



Besides the yascular and muscular impressions, there are others, in certain genera, 

 to which it is necessary in the next place to direct attention. The flat valve of 

 Productus often exhibits two reniform impressions or lobes, each one striking off from 

 between the pair of muscular scars situated on each side of the median plate (PI. XI, 

 fig. 10/; PI. XIX, fig. 3/), — an origin, which, to a certain extent, identifies them with 

 the primary vascular trunks, issuing in a similar way (and occasionally subdividing into 



1 Note sur le Magas pumilus ; Bull, de la Soc. Geol. de France, 2"" serie t. v, pi. ii. 



2 In Lingula the branchial system is somewhat modified. 

 "'' Transactions of the Zoological Society, vol. i, part ii. 



