Dr. J. D. Macdonalcl on Prohoscidiferous Gaderopodn, 113 



XVIIL — On the Homolugies of the Dental Plates and Teeth of 

 Prohoscidiferous Gasteropoda. By JoiiN Denis Macdonald, 

 M.D., F.ll.S., Staff Surgeon, R.N. 



[Plate XIII. ] 



All unisexual Gasteropoda furnished with a knigthy proboscis 

 retractile from the base, have also large single spherical otoliths 

 in the ear-sacs. The odontophore is ribbon-like, holding a 

 fixed relation to the extremity of the proboscis, whether re- 

 tracted or protruded ; and their lingual dental characters indi- 

 cate their division into two natural groups easily distinguished 

 from each other. 



In the first group the dental plates are arranged in seven 

 longitudinal series, and the teeth are in general recurved from 

 the anterior border of the plates — a character which is especi- 

 ally observable in the central and first lateral series, even 

 where the two outer members are in the form of simple curved 

 fangs. The buccal plates are generally well developed, and 

 exhibit some diversity of form and structure. A^ery little need 

 be said of the homologies of the dental plates and teeth of this 

 group ; for, with very few exceptions, resulting from suppres- 

 sion of one or two of the outer rows in the pleurte, the odonto- 

 phore is septiserial, and the corresponding parts in all the 

 genera may be readily recognized. Even in cases of suppres- 

 sion, as in Criocella and Laniellaria^ the remaining dental 

 organs are unequivocally fashioned like those of the more 

 perfect neighbouring genera. The rccmwature of the dental 

 processes, expressed by the word Campylodonta^ is the most 

 essential character of this section of Proboscidifera. 



In the second group, which is eminently carnivorous, the 

 dental processes of the central plates, and frequently also of 

 the first lateral series, point directly backwards, without re- 

 curvature properly so called ; and this being the distinguish- 

 ing feature of its members, I have applied to them the name 

 of Orthodonta. The eyes are variously situated on the outer 

 side of the tentacula, viz. near the tip, in the middle, at the 

 base, or on an external depressed lobe-like process. In some 

 the propodium is largely developed, either simple, as in Harpa^ 

 or divided into two lateral portions by a median sulcus, as in 

 Olli-a. But in most of the families the propodium is marked 

 ofi" from the mesopodium by a more or less deep trans^•crse 

 groove dilaminating the anterior border of the foot. 



The Orthodonta admit of division into two parallel sections, 

 distinguished respectively by the imcinate or the comb-like 

 character of the lateral tectli. Tlioiigh the prevailing form of 



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