114 Dr. J. D. Macdonald on the Homologies of the 



dentition is triserial, its derivation from one of a higher num- 

 ber is indicated by the occurrence of five members in the 

 tongue of Clavatula in the combed section, and in the tongue of 

 Olivella in the uncinate. On the other hand, the lateral teeth, 

 which are barely traceable in Harpa, are altogether suppressed 

 in Cymba and Marginetta, while the lateral fangs alone a«e 

 present in Mangelia and Pleurotoma. The examples of sup- 

 pression here noticed cannot be looked upon as equivalent to 

 primary types, though they may be quite characteristic of the 

 families in which they occur. The above remarks will be 

 better understood on carefully comparing the annexed outline 

 drawings of the leading forms of dentition occurring amongst 

 the Orthodonta. (PL XIII.) 



The lingual sac of Conus (fig. 1) presents a sigmoid flexure 

 about its middle, the teeth in the fore part being in general 

 directed forwards. 



The fangs are separately erected or depressed (I have not 

 accurately determined this point) by a special bundle of 

 muscular fibres, arising by a fine point in front of the articu- 

 lation of each, and being inserted by a kind of tendon a little 

 below a trochanter-like process at their base, reminding one of 

 the mode of insertion of the triceps muscle into the olecranon 

 process of the ulna. This arrangement, however, cannot 

 warrant the assertion that the teeth in the Toxifera, so-called, 

 are inserted into the fleshy proboscis. 



Notwithstanding the remarkable difference existing between 

 the long spiral shell of Terebra and the depressed, almost in- 

 volute, spiral form of Conus, the anatomy of the respective 

 animals is remarkably similar, exhibiting an obvious natural 

 affinity ; and the dentition of both is modelled upon the same 

 characteristic plan. 



The genuine Pleurotomce, which are notched in the outer 

 lip, will be found, on accurate comparison with Beta, Man- 

 gelia, or such shells as are grooved at the suture, to present 

 characters sufficiently well marked to suggest their separation. 



The lingual cartilages of Pleurotoma form two comparatively 

 large rounded masses, upon and between which the odonto- 

 phore lies evenly, without the sigmoid flexure of Conus or 

 Bela, but, on the contrary, it may be readily laid out quite flat 

 for microscopic examination. The odontophore of Pleurotoma 

 has very much the same relative proportions as that of Mitra ; 

 but the teeth are in two rows, long, smooth, rounded, tapering 

 and gracefully curved (as it were, to inaugurate the uncinate 

 series) . 



The Columbelhxke (PI. XIII. fig. 11), including the beautiful 

 little shells of the genera Nitidella and Conidea, seem to link 



