SUB-CLASS I STREPTONEURA 44] 
Sub-Class 1. STREPTONEURA. Spengel. 
(Prosobranchia, Cuvier ; Cochlides, Ihering.) 
Gastropods in which the visceral commissures are crossed, producing an 8-shaped 
loop , sexes separate; heart behind the gill; a shell almost always developed, and 
with few exceptions provided with an operculum. 
The Streptoneura, or Prosobranchiates as they are often called, constitute by 
far the largest group of Gastropods, and comprise at least 20,000 living and 
fossil species. The shell is usually spiral, more rarely symmetrical, saucer- 
shaped or conical. The intestinal sac is twisted from left to right, so that 
the anal opening is placed on the right side near the head, and the organs 
normally belonging to the right side (kidneys and gills) migrate over to the 
left. Asa rule, only one (the right) of the lamellar gills is fully developed, 
but in some cases the two are of equal size. The gill veins enter anteriorly 
into the single or double-auricled heart. 
The large number of Prosobranchiates have been variously classified. 
Cuvier, Milne-Edwards, and most of the older zoologists laid emphasis upon 
the number and formation of the gills; Troschel and Lovén upon the char- 
acters of the radula; Ihering upon the nervous system; Mérch and more 
recently Perrier and Bouvier upon the structure of the heart. As none of 
these characters leave a marked impress upon the shell, they are without 
practical value in Palaeontology. Nevertheless, the two orders Aspidobranchia 
and Ctenobranchia form natural groups, and are recognised, albeit under 
different names, in all classificatory systems. 
Order 1. ASPIDOBRANCHIA. Schweigger. 
(Cyclobranchia and Scutibranchia, Cuvier.) 
Nervous system not much concentrated anteriorly ; a penis generally absent ; radula 
' multiserial. 
This group includes most Palaeozoic Gastropods, and is regarded as the most 
primitive expression of the class. The nervous system and radula are of low, decidedly 
generalised type, and in some families two symmetrical ctenidia or gills are developed, 
as in Pelecypods. 
Sub-Order A. DOCOGLOSSA. Troschel. Limpets. 
(Cyclobranchia pars, Cuvier ; Heterocardia, Perrier.) 
Symmetrica l, with conte or bowl-shaped non-spiral shells ; operculum wanting. Organs 
of respiration represented either by a ring of laminae (secondary or pallial gills) beneath 
the mantle margin, or by a comb-shaped true gill in front, anterior to the heart, or by 
both true and secondary gills. Tongue set with peculiarly modified teeth. Heart with 
one auricle. Marine. Cambrian to Recent. 
The impression of the adductor muscle in the shell cavity is horseshoe-shaped, 
open in front. In the family Tryblidiidae, the horse-shoe is broken into numerous 
separate impressions. The three families Patellidae, Acmaeidac, and Lepetidae have 
the impression uninterrupted, and are distinguished by the structure of the gills. The 
