GASTEROPODA. | LOWER PALOZOIC MOLLUSCA. 289 
a more compact gland, farther back (in the apex of univalve shells) sending its vas deferens close along the 
oviduct. The heart is systemic, and has one auricle (or in Haliotis, Chiton, and Fissurella, two) receiving the 
oxygenated blood from the gills, which flowing into the ventricle, is thence distributed to the body through an 
aorta extending from its apex. The eyes never exceed two in number, but are perfectly formed, having a 
transparent cornea, a large, spherical, crystalline lens, with a small intervening space for the aqueous humour, 
and a circular pupillary opening left by the choroid or pigmental layer. The organ of hearing consists of two 
round vesicles, adhering to the anterior cesophageal ganglia, and containing numerous calcareous, oscillating 
otolites. The shell varies greatly in form and position, according to the group, but is formed in all from the 
calcification of cells formed in layers under the epidermis, by the edge of the mantle, and irregularities in form 
of its edge, produce corresponding spines, tubercles, &c. on the shell, which the animal is able to absorb again 
when they become internal by the spiral growth of the whorls: there are usually three distinct layers in the uni- 
valve shells. All the orders are hermaphrodite except the Pectinibranchiata, in which the sexes are distinct. 
The nervous system is greatly more centralised than in the Lamellibranchiata, and the principal ganglia ap- 
proximated to the head, there being usually a great pair of supra-cesophageal ganglia or brains, supply the 
eyes, tentacles, &c., and a great sub-cesophageal mass of ganglia, giving off nerves to supply the foot, the gills, 
the viscera, &c., connected by cords forming a nervous collar. 
The class is divided into several orders, chiefly from consideration of the breathing organs, viz. Ist, Pul- 
monaria, including all the air-breathing Gasteropods inhabiting the land or fresh water (never in the sea), and 
in which a vascular cavity at the side of the neck forms a sort of lung, fit for breathing air directly ; 2nd, Nudi- 
branchiata, composed of naked forms (not protected by a shell), having foliated naked gills, grouped in a rosette 
on the back or thereabouts ; 3rd, Znferobranchiata, composed also of naked genera, but with the branchize at the 
lower part of the sides of the body, between the foot and the mantle; 4th, Cyclobranchiata, having the gills 
encircling the body between the foot and the mantle, and covered by a simple conical shell (Patella, &c.), 
or by several transverse, imbricating, caleareo-corneous pieces (Chiton, &e.*) ; 5th, Tectibranchiata, having the 
gills concealed by a fold of the mantle, containing a rudimentary shell, or covered by reflected process of the 
foot (Bulla, &e.); 6th, Pectinibranchiata, or most highly organized of the class, having pectinated gills, usually 
placed in a wide cavity, and the body protected by a spiral univalve shell. This latter group includes nearly all 
the marine spiral univalves, and is here taken as including also the Twbulibranchiata, in which the shell is 
tubular and irregular, and Scutibranchiata, in which the shell is very wide and often depressed and simple 
(Fissurella, Haliotis, &e.); 7th, Dentalinee, in which the shell is tubular and the gills forming two bunches, 
one on each side of the neck. 
6th Ord. PErcTINIBRANCHIATA. 
This order contains a very large number of shells, which are” principally marine, a few being fresh-water 
and fewer still terrestrial. The order is divided into the following families :—Ist, Fisswrellidw ; 2nd, Crepidu- 
lide ; 3rd, Vermetide ; 4th, Buccinide; 5th, Casside ; 6th, Fuside; 7th, Muricide ; 8th, Volutide ; 9th, 
Conide ; 10th, Strombide ; 11th, Olivide ; 12th, Cypreide ; 13th, Lanthinide ; 14th, Haliotide ; 15th, Tro- 
chide ; 16th, Neritide ; 17th, Naticide ; 18th, Acteonide ; 19th, Pyramidellide ; 20th, Paludinide ; 21st, 
Ampullaride ; 22nd, Cyclostonide. 
2nd Family. CREPIDULIDZ. 
Shell wide, depressed, conoidal, with a slightly developed internal spire or none ; apex simple, or slightly 
inrolled toward the posterior end; mouth very wide, irregularly undulated in the adult by the inequalities of 
the surface, to which it permanently adheres: (animal with a wide, rounded foot ; mantle surrounding the shell, 
leaving a cavity at the head, in which the pectinated gills are lodged; tentacles wide, short). 
Genera :—1, Capulus; 2, Infundibulum. 
* Most probably Chiton, from its circulation, bilateral disposition of sexual organs, &c., does not belong to the Mol- 
lusea, in which case the anomalous structure of the covering would not disturb the uniformity of the remaining cyclobranchs. 
[ Fasc. 11.] Pp 
