Dr. J. D. Macdonald on Proboscidiferous Gasteropoda. 113 XVIII. — On the Homologies of the Dental Plates and Teeth of Proboscidiferous Gasteropoda. By John Denis Macdonald, M.D., F.R.S., Staff Surgeon, R.N. [Plate XIII.] All unisexual Gasteropoda furnished with a lengthy 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. Very 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 pleurae, 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 Lamellaria, the remaining dental organs are unequivocally fashioned like those of the more perfect neighbouring genera. The recurvature 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 Oliva, But in most of the families the propodium is marked off from the mesopodium by a more or less deep transverse groove dilaminating the anterior border of the foot. The Orthodonta admit of division into two parallel sections, distinguished respectively by the uncinate or the comb-like character of the lateral teeth. Though the prevailing form of Ann. &Mda.N. Hist. Ser.4, Vol. iii. 8
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