20 LECTURES ON MOLLUSCA. 



The Phragmoceras and Oncoceras form a sub-family, in which the 

 shell is pear-shaped and contracted at each end. 



The bent forms constitute another sub-family, and were perhaps more 

 nearly related to the Nautilus. Cyrtoceras is slightly curved, and 

 shaped like a gigantic Caecum.* Gyroceras dev elopes a shape like 

 Spirula ; and Ascoceras displays a shell bent upon itself, like PtycJio- 

 ceras among the Ammonites. 



Family 



In the living Nautilus, the only interpreter of the great group of 

 Tentacular Cephalopods (as D'Orbigny calls the order) the horny beaks 

 are surrounded with shelly matter, giving them great crushing power 

 over the shells of crustaceans. Similar beaks have been found fossil 

 in various strata, associated with Nautili. In the Muschelkalk of Ba- 

 varia, where there is only one species of Nautilus, the upper beak has 

 been described as Eliyncolites hirundo, and the under beak as " Con- 

 chorliyncus avirostris." D'Orbigny has turned these mandibles into 

 cuttle bones, under the names of JRJiyncoteuthis and Palceoteuthis ; one 

 out of the many instances in which a knowledge of comparative anat- 

 omy is shown to be essential to the study of organic remains. Kound 

 the mandibles is a circular fleshy lip ; round which again are about 

 four dozen labial tentacles, answering to the " buccal membrane" of 

 the cuttles, and serving to bring the prey to the mouth. Beyond these 

 are a double series of tentacles, thirty-six in number, answering to the 

 ordinary arms of the cuttles. When the creature is expanded for 

 crawling or seizing prey, these would project somewhat in the form of 

 a figure 8, the mouth being between the two groups of tentacles. 

 When the creature retires into its shell, it protects the opening with a 

 hood, which answers to the back pair of arms, united together and 

 developed for that purpose, as are one pair in the female Argonaut to 

 envelop the shell. The tentacles shut up in bunches into sheaths, 

 which correspond to the eight common arms of the cuttles. Besides 

 these there are four tentacles, one on each side of each eye : these appear 

 to be feelers as in the Gasteropods. It is easy to see how much more 

 highly organized and active is the paper, than its distant relative the 

 Pearly Nautilus. In each case, all the animals examined have been 

 females. It has been supposed that the shell-forms with a wide open- 

 ing at the axis of the spire, belong to the males, which, as in the other 

 Cephalopods, are few in number. Similar differences are found in 

 almost all the Ammonites. 



The Fossil Nautili present several sections, differing more or less in 

 type from the recent species. In Cryptoceras, the siphuncle is nearly 

 external, as in the Ammonites, which it resembles in external form. 

 In Temnocheilus, the shell is carinated. In Discites all the whirls are 

 exposed and flattened. These sections are from the palaeozoic rocks. 

 The " Ellipsolithes" were simply Nautili and Ammonites which had 

 been accidentally compressed into an oval shape. 



' The Corniculma figured by Munster as a chambered shell, is probably only a badly 

 observed Caecid. 



