MORPHOLOGY OF THE JJ ELTCINID^:. 797 



recognizable marks of distinction between diiferent species, 

 give a totally difterent I'esult. Taking these as a criterion, 

 E'Utrocliatella stands well apai't : the remaining genera show a 

 snfficient amount of similarity to justify our placing them in a 

 single group, in wliich Palcfohellcina stands nearest to Alcadia; 

 Aphanoconia is closely related to Pcdcvohelicina ; Oroho2)hana, 

 while showing relationship to the two last named, has distinctive 

 characters which keep it somewhat apart ; and Liocidella, while 

 showing relationship to Alcadia, has undei-gone modifications 

 which, in one feature at an}' rate, resemble those which distin- 

 guish Orohophana from Palceohelicina. 



Oa the whole, the radular characters aftbrd the safest clue to 

 affinity, agreeing as they do with the conclusions founded on 

 conchological characteis and on geographical distribution. 



In all the Helicinida^, so far as is known, the radular formula 

 may be represented as oo . 1(3 + 1 +3) . 1 . oo . The Proserpinidfe 

 have a somewhat difierent, and the Hydrocenidse a very different, 

 formula, biit they need not be considered here. 



As different authors use different names in describing the teeth 

 of Neritoid gasti'opods, I must define my tei-ms before proceeding 

 further. Of the teetli included in brackets in the formula, I call 

 the single tooth in the centre the median, ; the three teeth on 

 either side of it the adinedians. The large tooth on either side of 

 the admedians 1 call the lateral, and the numerous teeth to the 

 outside of these the marginals. In Eutrochatella, as Troschel has 

 shown, the lateral teeth are relatively very large and of charac- 

 teristic shape, being mushroom-shaped, with the top of the pileus 

 hollowed out to form an articular cavity, into which fits the stalk 

 of the lateral tooth of the row next in front of it. The edge of the 

 pileus is entire. I have attempted in fig. 57 (PI. XL.) to give 

 some idea of the veiy complicated structure of this tooth in Eutro- 

 chatella fulchella. It should be noted that it has an external 

 process, or " Basalanhang," which Troschel described as charac- 

 teristic of the genus Helicina. For the rest, this mushroom-shaped 

 tooth more nearly resembles in shape the corresponding radular 

 tooth in the Neritidse than is the case in any other Helicinid, 

 The mai'ginal teeth of Eutrochatella are simply pointed curved 

 bars, without denticulations at their free extremities. In the 

 genus Helicina Lamarck [nan sensii restricto Wagner) the laterals 

 are not pileiform, but consist of a stout median portion which I 

 shall call the " stalk" ; from the inner side of this a mox-e or less 

 broad aliform plate projects obliquely forward ; the anterior 

 border of this plate is thickened, recurved, and bears a number of 

 denticulations, varying from 7 to 12 in number in the diffei-ent 

 species that I have studied. Attached to the outside of the stalk 

 by an imperfect joint is the pointed external process (the 

 " Basalanhang " of Troschel), and the top of the stalk is excavated 

 to form an articular cavity for the hinder end of the stalk of 

 the corresponding tooth in the row next preceding. Also, in all 

 species of Helicina the marginal teeth have broadened recurved 



Proc. ZooL. See— 1911, No. LV. 55 



