248 MOLLUSCA. GASTEROPODA 



a deep slit near the suture ; inner lip smooth. Anterior canal long, 

 straight, narrow. Operculum horny, ovate, acute, nucleus apical. 

 Upper Cretaceous to present day. Ex. P. babylonia, Eecent ; P. 

 undata, Bracklesham Beds. 



ConUS. Shell conical, generally smooth, the last whorl 

 enveloping the greater part of the preceding whorls. Spire short, 

 flattened or conical, with many whorls. Aperture long, narrow, 

 straight, with parallel or sub -parallel borders, ending anteriorly 

 in a truncated canal ; outer lip thin, simple, no folds or teeth, 

 notched at the suture. Columella straight, smooth. Operculum 

 horny, much smaller than the aperture. Upper Cretaceous to 

 present day. Ex. C. marmoreus, Recent ; C. deperditus, Brackle- 

 sham Beds. There are numerous sub-genera. 



The Heteropoda are a group of the Streptoneura which 

 have become modified for a pelagic mode of life. The 

 foot is laterally compressed so as to form a vertical fin. A 

 shell may be absent, but, when present, it is always thin 

 and light. Only a very few forms have been found fossil. 



ORDER II. EUTHYNEURA 



The visceral nerve-cord forms (except in a few genera, 

 e.g. Actceon) a simple, untwisted loop. One gill only is 

 present. An operculum is generally absent. There are 

 two sub-orders, (1) Opisthobranchia, (2) Pulmonata. 



SUB-ORDER I. OPISTHOBRANCHIA 



The gill is placed behind the heart ; there is one 



auricle only, which is behind the ventricle. All the 



Opisthobranchia are marine ; they are divided into two 

 groups : — 



(1) the Nudibrancliia, in which there is no mantle, 

 and no shell in the adult. No examples of this division 

 have been found fossil. 



