10 MR MACDONALD ON THE ANATOMY 



quently becomes plan-orbicular, laterally compressed, and nearl}' symmetrical, 

 ])earing a dorsal keel of variable depth, literally nothing more than a thin fold of 

 the shell itself, which thus presents a deep notch or slit at the corresponding 

 part of the outer lip. 



The animal admits of complete retraction within its shell, and is furthermore 

 protected by a delicate oval operculum, with a large sinistrallt/sipiral median, and 

 subapical nucleus (Plate II. fig. 1). 



The head of Atlanta is supported by a kind of neck, with which the proboscis 

 forms an angle more or less obtuse. The eyes are proportionately very large, 

 and fronted by small conical tentacula ; and all the parts of the foot are more 

 compact than in Firoloides. Thus, the part corresponding with the tail in the 

 latter genus is an obvious metapodium, bearing an operculum in the former. 

 The swimming-plate is relatively stronger, and the sucker-disc better defined. 

 The visceral mass also bears a larger proportion to the rest of the body. 



More Particular Description of the Foot and Retractor Muscle. 



The muscular fibres which fix Atlanta to the shell arise from a short oblique 

 line commencing near the nucleus, and extending some little distance outwards 

 and forwards on the upper or right wall of the tube. From this origin they pass 

 round the columellar wall, gradually diverging from one another as they approach 

 the mouth of the shell. Here they commence to blend with the proper muscular 

 fibres of the foot, but more especially with those of the operculigerous lobe. This 

 lobe forms the posterior part of the foot, from the inferior surface of the base of 

 which the vertical fin bearing the sucker-disc springs. Thus the foot altogether 

 may be said to consist of three distinct portions— viz., the swimming-fin (Plate II. 

 fig. 1, j) in front, the operculigerous lobe (Plate II. fig. 1, 0) behind, and the sucker- 

 disc (Plate II. fig. 1, k), holding an intermediate position. These structures are 

 mainly composed of a basis of muscular and areolar tissue, overlaid with common 

 integument. The muscular fibres are disposed in ribbed bundles at the central 

 parts requiring the greatest support, and cross one another diagonally over the 

 general surface, so that mobility in every direction is amply provided for; but as all 

 the parts of the foot lie in one vertical plane, corresponding with that of the shell, 

 all the lateral movements of the animal are necessarily the more vigorous. 



The fin-shaped swimming-plate supports the mesopodium or sucker-disc on its 

 posterior border near the base. The latter disc is divided by a longitudinal 

 depression into two lateral lips or lobes, which meet together when the organ is 

 contracted ; but w^ien it is fully expanded and brought into action, after the 

 manner of its homologue, the creeping disc of the true gasteropod, it exhibits a 

 more circular form, and as I have particularly observed in the case of Oxy gyrus. 

 o-lides over any resisting surface with imperceptible undulations. Into its central 

 or concave part a muscular fasciculus, derived from the fibres of the great retrac- 



