84 The MoUuscan Family Planorbidae 



part of the tooth. The first thirty-four rows measure 5.4 microns, rows 35 

 to 89 measure 6 microns and rows 90 to 109 measure 5.4 microns. All 

 central teeth examined exhibited this variation in width. 



In riisei (plate 68, fig. 6) the formula is 18-1-18 with 105 rows of teeth. 

 The center tooth is like that of havanensis but the cusps do not reach the 

 base of attachment. There are four lateral teeth. The 5th and 6th teeth are 

 intermediate with interstitial cusps between the entocone and ectocone and 

 small cusps above the ectocone (figs. 7, 5, 6). Seven to twelve are typical 

 marginals with a variable number of small cusps, as in havanensis. The 

 15th to 18th teeth are vestigial. The center teeth vary in width as in 

 havane7isis, but the teeth are somewhat smaller. 



In obstructus (plate 68, fig. 7) the formula is 19-1-19 and there are 124 

 rows of teeth. There are seven tricuspid laterals, the 8th tooth is inter- 

 mediate, modified only on the entocone, and there are nine marginals 

 becoming narrower toward the edge of the membrane. The outer marginals 

 are vestigial. The center tooth varies in width as in the other species. 



H. B. Baker (1930, plate 28, fig. 8) figures the radula of pallidus from 

 Bolivar, Colombia. The formula is 18-1-18 with five laterals and there are 

 106 rows of teeth. The radula of kiihncrianus is described on page 47 of 

 the same paper. The formula is 19-1-19, with six laterals and 106 rows of 

 teeth. Thus the radulae of five species of Tropicorbis are known; havan- 

 ensis, which is nearest to orbiculus i^licbmanni) , the type of the genus, 

 obstructus, riisei, pallidus, and kiihnerianiis. These cover fairly well the 

 different variations of the species of the genus. 



The material personally examined is as follows: 



havanensis. New Orleans. Collected by Dr. E. C. Faust; 10 specimens 



riisei. Barceloneta, Puerto Rico. Collected by Dr. W. A. Hoffman; 12 specimens 



obstructus. New Orleans. Collected by Dr. E. C. Faust ; 10 specimens. 



Geographical Distribution. The genus Tropicorbis is one of the most 

 widespread of any group of American Planorbidae. From Louisiana and 

 Texas in the United States, it extends southward through Mexico and 

 Central America into South America as far south as the Argentine Re- 

 public, a distance of about 7.000 miles. The group is also found in many 

 of the islands of the West Indies. In number of species represented, it out- 

 ranks any other group of the family found in America except Hclisoma, 

 upward of thirty-three species and races being now recognized (see a later 

 page for the list of species). 



Geological Distribution. Oligocene or Lower Miocene to Recent time. 



Remarks. The genus Tropicorbis presents some puzzling features of its 

 anatomy which render its systematic position debatable. The nature of the 

 prostate, with many diverticula in a single row, on a separate prostate 

 duct, places the genus in the subfamily Planorbinae. The kidney is also 

 like other members of this subfamily. The penial complex differs from that 

 of Anisus and Gyraulus in lacking a penial stylet. The jaw is quite differ- 

 ent and resembles this organ in Helisomatinae and Planorbulinae. The 

 teeth of the radula also differ in having interstitial cusps on the inter- 

 mediate and marginal teeth. Pilsbry (1934, p. 53) remarks that the group 

 resembles Gyraulus but differs in the unarmed verge and the different jaw 

 and marginal teeth. All points considered, it would seem that its place is 

 with the Planorbinae, the largest number of characteristics agreeing with 

 those of this subfamily. 



