Mollusca — Pteropoda. 43 



Ammonites in whicli the animal could retreat wholly within the GALLERY 

 hody-chamber of its shell. It was secreted by the coalesced pair 

 of dorsal arms, and corresponds with the thick muscular hood which cases 62, 

 closes the mouth of the shell in the living Nautilus. Aptychi 65-68. 

 occur in the Chalk, the Kimeridge Clay, the Solenhofen beds, the 

 Oxford Clay, the Inferior Oolite, and Lias (see Pigs. 79a, h). 



Ptekopoda, etc. — The Pteropods (" Sea Butterflies " or " Winged 

 Snails ") are found swimming near the surface in the open sea, 

 the living forms being all of small size. They have no distinct 

 head ; the mouth is placed anteriorly in the centre of the forepart 

 of the foot, which is often rudimentary, but may be drawn out 

 into one or more pairs of tentacles and provided with suckers. 

 The lateral parts of the foot are expanded into a pair of wing- 

 like muscular lobes, which are used as paddles. The hind 

 portion of the foot is often rudimentary, but may carry an 

 operculum. The Pteropods are divisible into two sections, the 

 Gymnosomata and the Thecasomata] in both sections the embryo 

 is provided with a shell, but in the Gynmosomata this is soon last 

 and the adult is devoid of any shell, while the adult members 

 of the other section {Thecosomata) generally possess a very delicate 

 external calcareous shell, of which the embryonic shell usually 

 forms the initial portion. In the majority of the Pteropods the 

 shell is symmetrical, but in some {Limaeimdce) it is coiled into 

 a spiral. The Pteropods are all hermaphrodite. They have been 

 heretofore regarded as a distinct class of the Mollusca, but recent 

 investigations show that their internal organization does not 

 essentially differ from that of certain of the Gasteropoda. 



Indubitable Pteropods are found in the Tertiary deposits,^ but 

 the Mesozoic rocks have not yielded any true Pteropods, and 

 it is not until we come to the Devonian and Silurian strata that 

 forms closely resembling the recent genus Styliola {Creseis) are 

 met with; the Pteropodal nature of these has, however, been 

 disputed. These delicate calcareous shells have a conical form, 

 and no partitions ; they occur in abundance in some of the Devonian 

 rocks of North America. 



Three important Palaeozoic genera, Tentaculites, SyoUthes, and Table- 

 Conularia, have often been grouped with the Pteropoda. The first 



^ The undoubted Tertiary Pteropods have been removed from this case and 

 placed with the other Foreign Mollusca in Wall-cases 1-4 in Gallery VIII. 



