CEPHALOPOD ADAPTATIONS 



99 



ceratidae, and Cyrtendoceratidae; and every degree of 

 internal filling of the earlier chambers may be seen 

 in the families Actinoceratidae, Jovellanidae, and 

 Poterioceratidae — collectively covering the time 

 from Ordovician to Devonian. 



An equally effective method of main- 

 taining horizontality is illustrated by the 

 families Ascoceratidae and Mesoceratidae 

 — constituting the sub-order Mixocho- 

 anites of Hyatt — Ordovician and Silurian 

 nautiloids. In these the shells have 

 advanced slightly beyond the orthoconic 

 form in most of the genera and are more 

 or less cyrtoconic. The living chamber 

 of the adult is long and inflated, often 

 contracted at the aperture, which is some- 

 times crested. The manner in which the 

 center of gravity is kept back is by the 

 formation of a linear series of air chambers 

 (saddles) along the dorsal wall of the 

 living chamber. Two examples of this 

 are shown in the accompanying figures 

 (Plate 3) : Proterocameroceras — an orthoconic 

 form, and Ascoceras — a cyrtoconic form. 

 A somewhat similar arrangement is ex- 

 hibited in the allied family Piloceratidae 

 of the Ordovician. In Piloceras the shell 

 is relatively short and wide; the animal 

 was stout, its visceral cone extending 

 backward over six-sevenths of the distance 

 to the apex of the shell; the living cham- 

 ber is restricted dorsally and the animal 

 thus rendered buoyant by the development 

 of numerous dorsal saddles (air chambers), 

 and the apex of the shell is ballasted by 

 pseudosepta and conchy olin endocones. 



According to Geikie, (I have not taken 

 the pains to verify his count) the Silurian 

 of the Bohemian basin furnished Barrande 

 with 1 12.7 species of Cephalopoda, Of 

 these 554 were orthocones, and other 

 statistically minded students have esti- 

 mated the number of orthocones in the 

 Ordovician as approximately fifty per cent 

 of the total number of species of Cephalo- 

 poda known from that period. The 



percentage is somewhat less for the 

 Silurian as a whole, about Z5 per cent for 

 the Devonian, and about zo per cent for 

 the Carboniferous as a whole, although 

 some very large sized forms do survive as 

 late as Carboniferous times. In post 

 Triassic times there are no orthoconic 

 nautiloids. This proves that the general 

 course of evolution in this sub-class was 

 as I have indicated, and effectually com- 

 bats the seemingly eccentric view of Owen 

 — in which strangely enough he is 

 followed by Willey and Spath — that the 

 orthoconic are uncoiled from whorled 



ancestors. 



THE CYRTOCONES 



The number of species of cyrtocones in 

 the Silurian of the Bohemian basin was 

 330. Both orthocones and cyrtocones 

 were all originally referred to the two 

 comprehensive genera Orthoceras and 

 Cyrtoceras, but later systematists have 

 partially segregated them into more nat- 

 ural generic groups. I have already 

 indicated a probable variety of habits 

 among the orthocones, and this is quite 

 as clearly indicated among the cyrtocones. 

 There can be slight doubt that what might 

 be called the normal cyrtocone was de- 

 rived from orthoconic ancestors in the 

 manner already outlined, or that such a 

 one as is shown in Plate z, figure 5 was a 

 horizontal swimmer. 



Among the cyrtocones, however, there 

 are a considerable number of so-called 

 breviconic forms, all from the Silurian 

 (Etage E), which it seems to me throw an 

 important light on their structure and 

 habits. They are all of about the same 

 size, i.e. 4 to 5 centimeters long and z to 

 3.5 centimeters in maximum diameter, 

 and hence justly called small forms, and 

 are conspicuously marked by mostly 

 transverse color patterns that entirely 

 encircle the shell and prove conclusively 



QTTAB. REV. BIOL., VOL. Ill, NO. 1 



