MOLLUSC A. GASTEROPODA. 155 



8. Convolute; when the last whorl covers all the 

 others and the aperture is consequently as long as the 

 shell, as in Cyprcea, 



9. Auriform; aperture very large and spire very 

 short, as in Haliotis. 



The surface of the shell is frequently ornamented with 

 spines, or with ribs and striae, these are said to be 

 longitudinal when they run parallel to the sutures from 

 the aperture to the apex ; they are transverse when they 

 run across the whorls from suture to suture. When rows 

 of spines or processes extend across all the whorls from 

 the apex to the base of the shell, as in Murex, we have 

 what are termed varices. The colours with which the 

 shells of recent gasteropods are provided are only pre- 

 served fossil in rare cases. The shell consists of an outer 

 epidermal layer, and of a thick porcellanous layer; in 

 some cases there is also an inner nacreous or pearly layer. 



The Gasteropoda are divided into the following six 

 orders, (1) Polyplacophora, (2) Prosobranchiata, (3) Opis- 

 thobranchiata, (4) Heteropoda, (5) Pulmonata, (6) Ptero- 

 poda. 



OEDEE. POLYPLACOPHOEA. 



The body is bilaterally symmetrical, and the shell is 

 formed of eight calcareous plates. This includes the 

 genus Chiton and allied forms. The order is only poorly 

 represented in the fossil state ; the earliest representatives 

 occurring in the Ordovician rocks. 



OEDEE. PEOSOBEANCHIATA. 



The gills and auricle are in front of the ventricle of 

 the heart ; there is always a shell. 



