General Morphology 11 



of the side of the horseshoe, showing that tlie hiteral jaws are inokidcd in 

 the segmentation (see plate 50, fig. 16, Gijraidus parvus). 



A peculiar jaw, not included in the usual segmented type of the family, 

 is that of Cariuife.T in which the jaw is b()w-shai)ed (or roundly horseshoe- 

 shaped) and is composed of many narrow plates fastened to a heavy 

 cartilage (plate 49, fig. 19, Carinifex ponsonbyi; fig. 18, Carinifex jackson- 

 ensis) . Althcnigh belonging to the subfamily Helisomatinae the jaw is prac- 

 tically of the Planorbinae type. Figure 17 on plate 49 shows the large size 

 of the cartilage in this genus (C. iacksonensis) . 



This type of segmented jaw is suggestive of the polyplacognath group 

 of land snails. It is said to be a very ancient type of jaw which would seem 

 to indicate that the Planorbinae group of the family is more ancient than 

 the Helisomatinae group (except Carinifex which has the primitive form of 

 jaw). The marginals of the radula also indicate a primitive form of radula. 



[2] The Radula 



The radula of the Planorbidae is ribbon-like as in the Lymnaeidae. It 

 lies on a subquadrate cartilage near the rear end of the buccal sac. The 

 front end of the radula extends toward the mouth of the snail and the rear 

 end is bent downward into a radular sac which forms a rounded protrusion 

 or bump at the back of the buccal sac (see plate 70, figs. 12, 13). The 

 radula grows forward from this sac as the functional teeth at the front 

 end of the ribbon become worn by use. There are protractor and retractor 

 muscles which iiull the radula backward and forward, over its cartilage, 

 during the act of cutting u\) food particles. In use, the radula is moved 

 from behind forward, like the tongue of a cat lapping food from a plate, 

 as described by Sterki many years ago. 



The lingual ribbon of the Planorbidae carries many teeth in transverse 

 rows as in the family Lymnaeidae. There may be as many as 200 rows 

 with 85 teeth in a row or a total of 17,000 teeth in one ribbon. The central 

 tooth is always bicuspid and in some groups (as in Drepanotrema) there 

 are one or more accessory cusps on either side of the central cusps. There 

 is a series of duplicating teeth on each side of the central tooth. This row 

 is divisible, as in the Lymnaeidae, into lateral, intermediate, and marginal 

 teeth. The lateral teeth are usually tricuspid and comprise an inner, short 

 entocone, a large median mesocone, and a smaller outer ectocone (see plate 

 57, fig. 1). The cusps are usually dagger-like and may be wide or narrow. 

 The intermediates are those teeth between the typical lateral and marginal 

 teeth which show certain modifications, as splitting of the ectocone or 

 entocone into smaller cusps. The marginals are usually somewhat claw-like 

 with a variable number of small cusps. The mesocone usually persists in 

 the earlier marginal teeth and may be recognized by its large size and 

 central position. 



The law of mesometamorphosis applies to the Planorbidae as well as to 

 the Lymnaeidae. As stated by Pilsbry this law is as follows: 

 All modifications of the teeth proceed from the median line of the radula outwards 

 toward the edges, the outer marginal teeth being the last to be modified (Guide to 

 Study of Helices, p. xiii). 



The marginals of the Planorbidae are of the same ])rimitive type as are 

 those of the Lymnaeidae, showing a close relationshii) between these two 



