Subfamilies, Genera, and Subgenera — Recent and Fossil 145 



The buccal sac (BP) is a pear-shaped organ, 2 mm. long and 1.5 mm. 

 high. The oesophagus leaves the oral cavity near the upper posterior por- 

 tion of the pouch. There are two strong buccal retractor muscles (BR). 

 The salivary glands (SG) are two in number, 3 mm. long, which enter the 

 buccal cavity near the oesophagus by a long, narrow duct (1 mm. long) 

 on each side. The salivary glands form a loop, their posterior ends 

 connected. 



There is not much variation in the form of the buccal sac in Pierosoma. 

 In some species, as pilsbryi, corpulentum, tenue californiense, and sub- 

 crenatum (plate 70, fig. 12), the whole sac is a trifle more elongated, espe- 

 cially in the fore part. There is reason to believe, however, that the more 

 elongated aspect may be due to distortion in the animal when killed. At the 

 back end of all sacs there is a rounded or elongated radula sac (plate 70, 

 fig. 13, RL) which has to do with the formation of the radula membrane. 

 This varies in form in some of the species. 



The jaw in Pierosoma is always of three parts. The superior jaw is 

 oblong, wide and low, sometimes somewhat arcuated. The face of the 

 superior jaw is marked by fine striations which form a more or less 

 denticulate border to the jaw when seen with high magnification. There 

 are two long and narrow lateral jaws one on each side bordering the lips 

 of the animal, and usually seen in the form of a question mark. The jaw of 

 Helisoma trivolvis is shown on plate 49, fig. 2, and this is characteristic 

 of all members of the subgenus. The jaws of some other species are figured 

 as follows: 



truncatum, plate 49, fig. 3; pilsbryi, fig. 5; infracarinatum, fig. 8; corpu- 

 lentum, fig. 15; magnificujji, fig. 16; oregonense, fig. 6; subcrenatum, fig. 7; 

 horni, fig. 9; plexatum, fig. 10; traskii, fig. 12; occidentale, fig. 13; 

 californiense, plate 50, fig. 1. 



Radula. The radula of Pierosoma is similar to that of Seniinolina in 

 the bluntly rounded mesocone of the lateral teeth. The radula of the type 

 species, Helisoma trivolvis, is shown on plate 55, fig. 1. 



The center tooth has two spade-shaped cusps which do not reach the 

 lower margin of the base of attachment. The lateral teeth (1-10) have a 

 quadrate reflection which is tricuspid, the entocone and ectocone sharp 

 and spade-shaped, the mesocone rounded. The intermediate teeth (11-13) 

 have the reflection somewhat narrower and more elongated than the lateral 

 teeth, the entocone breaking up into three small, sharp cusps and the 

 ectocone region developing two or three cusps above the ectocone on the 

 outer margin of the tooth. Marginal teeth (14-18) still narrower, claw-like, 

 with the entocone broken up into four to five small cusps and the ectocone 

 into four or more small cusps on the upper, outer margin of the tooth. The 

 extreme outer marginal teeth are small, very narrow, and with slight 

 denticulations. There is some variation from the type described above, 

 some of the lateral teeth developing two cusps in place of the one ectocone, 

 as shown in fig. 4. 



The radulae of the races of trivolvis differ little from the type species. On 

 plate 55, several of these arc shown, lentum (figs. 6, 7) and pseudotrivolvis 

 (=lentum) figs. 2, 3. The radula of the large race, macrostomum, is shown 

 on plate 56. fig. 2. In chantaiiquerise (plate 55, fig. 5), the lateral teeth 

 (1-11) are narrower than in trivolvis. In winslowi (plate 56, fig. 1), the 



