170 LECTURES ON MOLLUSCA. 



rian and Carboniferous seas. In these, the sutures are not foliated, 

 hut simply lohed, often at sharp angles. In the ceratites of the Mus- 

 chelkalk series, the alternate lobes are denticulated. The Goniatite, 

 when the spire is unrolled injto a straight cone, like the Orthoceratites, 

 becomes a Bactrite; and the Ceratite, similarly unrolled, becomes a 

 Baculina. 



The true Ammonites, with minutely lobed septa, present all varieties 

 of shape ; from the compressed forms, with the whirls scarcely touch- 

 ing, to the involute species, with round backs, narrow chambers, and 

 very small umbilicus. They have been variously divided into groups 

 by different authors ; but they pass into each other by very slight dis- 

 tinctions. Often a shell, which in its .earlier stages would belong to 

 one group, develops into a different one as it approaches maturity. 



The Ammonites present various aberrant forms, some corresponding 

 to those already mentioned among the Nautili, some peculiar to them- 

 selves. In Crioceras the whirls are separate, as in Spirula. In 

 Scapliites, the shell begins like an Ammonite, the mouth is next pro- 

 duced at a tangent, and then bent back upon itself. It would be curi- 

 ous to know how such creatures got their living. Ancyloceras com- 

 bines the characters of the two last genera, beginning as Spirula, and 

 ending as Scapliites. Anisoceras has the same form, but drawn out of 

 the plane into an irregular spiral, like Vermetus. Toxoceras presents 

 a simple cycloidal curve. In Eamites, the shell begins cpuite straight, 

 then bends and returns again parallel to itself, and so on, like a Spi- 

 rula drawn out and flattened on its two sides. In the section Hamu- 

 lina, the shell only makes one bend, the two parallel limbs having 

 different sculptures, and the body-chamber occupying one limb and the 

 elbow. The Ptychoceras is like a Hamulina, with the two limbs 

 joined together ; still with different sculptures, so that fragments 

 might easily be described as distinct species. In Baculites, the shell 

 is quite straight, like a walking stick. It is so common in the Nor- 

 mandy chalk as to give it the name of Baculite Limestone. 



In the Terrilite group, we have an approach to the ordinary shape 

 of the univalve spiral shells. They are mostly reversed, and are sup- 

 posed by Woodward to have had one pair of gills atrophied. In 

 Heteroceras , after beginning as a Turrilite, the shell becomes separate, 

 as in the adolescent Vermetus, and makes an irregular spire eveloping, 

 but not touching, the spire. The Ilelicoceras is as it were a Turrilite, 

 with all the whirls drawn out into a corkscrew. 



We have now enumerated the principal known forms of Cephalo- 

 pods, both extinct and living. While they are the most highly 

 organized of invertebrates, they cannot be considered as typical mol- 

 lusks; that is, they do not represent the idea of molluscan life, as do 

 the ordinary Gasteropods which we have next to consider. Now those 

 classes which go off from the standard idea are generally pretty well 

 defined ; while those in which the normal idea culminates are more 

 variable in structure. We have seen that the cephalopods are all 

 formed on two well-marked but distinct types ; and however much the 

 shell of the Baculite may differ from the Nautilus, or the Argonaut's 

 egg-case from the cuttle-bone, a beginner even could never doubt con- 



