EAST COAST MARINE SHELLS 



19 



teeth upon each side. The radula being bi- 

 laterally symmetrical the last formula may 

 be abbreviated to read: 







Further study may reveal that the 

 cusps are denticulated or toothed, and 

 where there are no lateral or uncinal teeth 

 and the median is reduced to a tricuspid 

 series the condition is expressed: 



1 

 3 



: 







Drawings of the radulae are very 

 useful for reference. 



The radula affords aid in the dis- 

 tinction of genera and species. As a ba- 

 sis for classification it has been used in 

 dealing with the Gastropoda, particularly 

 the order PROSOBRANCHIATA. The latter, 

 when using this system, is divided into the 

 MONOTOCARDIA and DITOCARDIA. Dealing first 

 with the Monotocardia there are the fol- 

 lowing divisions: 



(a) TOXOGLOSSA. Three families, 

 all represented on the east coast, Tere- 

 bridae, Conidae, and Cancellariidae belong 

 to this group. The radula consists merely 

 of large marginal teeth on each side, no 

 central tooth and no laterals. In Conus 

 the teeth attain great size and are provid- 

 ed with a poison gland. 



(b) RACHIGLOSSA. It includes among 

 others the Ollvidae, Marginellidae, Volutidae 

 (Fig. 24), Mitridae, Pyrenldae, Muricidae, 



(Fig. 21) Coralliophilidae. 

 Most or all of these have 

 been or are carnivorous. 

 The radula consists of a 

 central tooth with one to 

 fourteen cusps, a single 

 lateral with more or less 

 cusps, the outer being the 

 largest. Teeth mostly 

 sharp, hooked and with a 

 broad cutting edge. In 

 Fasciolaria they are 

 toothed like a comb. 



(c) TAENIOGLOSSA. 

 Fig. 24 Here are grouped about 

 Radula of Voluta forty-six families includ- 

 ing the Cassldidae, 

 Cypraeidae, Strombidae, 



Fig. 26 

 Radula of Littorlna 



Fig. 25 

 Radula of Murex 



Cerithidae, Littorinidae (Fig. S6) , and 



Naticidae. The central tooth is very vari- 

 able, usually 

 multicuspid, 

 the central 

 cusp being dom- 

 inant; a single 

 lateral, more 

 or less cusped; 

 two uncini, 

 singly hooked 

 or a little 

 cusped. The 



normal formula of the Taenioglossa is 



2.1.1.1.2. 



(d) PTENOGLOSSA. This group in- 

 cludes the Janthinidae and Epitoniidae. The 

 radula possesses an Indefinite number of 

 hooked teeth, the outside being the larg- 

 est. In Janthina the central tooth is ab- 

 sent, the ribbon being of two large divi- 

 sions with a gap between them down the cen- 

 ter. 



(e) GYMNOGLOSSA. Both jaw and radu- 

 la being absent it is difficult to identify 

 the two families, Melanellidae and Pyra- 

 midellidae, which are placed here. 



Under the Diotocardia, already re- 

 ferred to, there are three groups: 



(f) RHIPIDOGLGSSA. About seventeen 

 families are included, among them the 



Neritidae, Turbini- 

 dae, Trochldae, and 

 Fissurellidae (Fig. 

 27) . The radula is 

 remarkable in the de- 

 velopment of the 

 Fig. 27 uncini ot- outer teeth. 

 Radula of Fissurella They are long, hooked, 



often cusped, ar- 

 ranged like a fan's ribs, curving backward 

 as they diminish in size. In only a few 

 cases are they countable. The average num- 

 ber of lateral teeth is five. In the 

 Neritidae the lateral is especially large 



