SDB-CLA>SI STREPTONEURA 441 



Sub-Class 1. STREPTONEURA. Spengel. 



(Prosobi'anchia, Cuvier; Cochlides, Ihering.) 



Gastropods in which the visceral commissures are n-nw,/, {</, K-iuy an 8-sJij>l 

 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. As a 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 Loven upon the char- 

 acters of the radula ; Ihering upon the nervous system ; Morch 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, an-1 in ><>me families two symmetrical ctenidia or gills are developed, 

 as in Pelecypods. 



Sub-Order A. DOCOGLOSSA. Troschel. Limpets. 

 (Cyclobranchia pars, Cuvier ; Heterocardia, Perrier.) 



Symmetrical, ivith conic 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, Acmaeidae, and Lepetidae have 

 the impression uninterrupted, and are distinguished by the structure of the gills. The 



