306 INVERTEBRATE MORPHOLOGY. 



groiip only one is present, while in Patella botli have disap- 

 peared, their place having been taken by respiratory folds 

 of the mantle. Both kidneys are invariably present. 



The primitive character of the suborder is further shown 

 in the absence of certain structures found in more specialized 

 forms. Thus the foot is flat and undifferentiated into pro-, 

 meso-, and metapodium ; the anterior part of the digestive tract 

 is not evertible as a proboscis ; there is no siphonal prolonga- 

 tion of the mantle, and no notch or siphonal groove on the 

 margin of the shell ; and there is no penis. On the other hand 

 the epipodium is usually well developed, as are also the pedal 

 nerve-cords, which are connected by numerous cross-commis- 

 sures. 



A further distinguishing feature of the suborder is the 

 arrangement of the teeth of the radula. Each transverse row 

 of teeth presents an indefinite number of marginal teeth, usu- 

 ally a single lateral, a single median, and a varying number 

 of admedian teeth, an arrangement known as rhipidoglossate. 

 Thus in Haliotis the arrangement is indicated by the formula 



Fig. 136.— Dentition of rroe/ms (after i.ankester). 



X, 1, 5, 1, 5, 1, X ; in Fissurdla by x, 1, 4, 1, 4, 1, as ; and in Tro- 

 chus (Fig. 136) and Turbo by x, 0, 5, 1, 5, 0, x, the single lateral 

 tooth being absent in these forms. In Patella, however, an- 

 other arrangement is found characterized by the occurrence 

 of only a small number of marginal teeth and by the absence 

 of the median, the formula being 3, 1, 2, 0, 2, 1, 3 ; this ar- 

 rangement is termed docogiossate. 



3. Suborder Monotocardia. 



In this suborder the effect of the pressure of the visceral 

 hump on the organs of the left side of the mantle-cavity is more 

 pronounced than in the Diotocardia. The heart possesses a 



