98 HoGG^ on the Lingual Membrane of Mollusca. 



making a close examination we find, in by far the larger 

 number of the Gasteropoda, one or more central or median 

 teeth,* with a certain number of laterals, diverging in nume- 

 rous rows on either side. Some species have, besides, one 

 or more horny mandibles, and even an additional buccal 

 plate, sometimes armed with minute spines. 



The horny mandibles of the mollusca are certainly de- 

 serving of more attention than they have received, with a 

 view to the elucidation of tlieir affinities. " The mandible is 

 a median plate attached to the bulbus pharyngeus over the 

 oral aperture, serving to divide and pound up the food." So 

 far as I have been able to make out, there are three, if not 

 four, different kinds of mandibles or maxillae. 1st. Those 

 divided by a median articulation into two equal parts, and 

 covered with fine, acute spiny processes placed in regular 

 rows throughout, as in Cyclotus. 2nd. The horseshoe 

 shaped, with a corrugated or sulcated arrangement, chiefly 

 found in the inoperculata. And 3rd. The smooth, beak- 

 shaped mandible, belonging to Cephaloj)oda. I believe there 

 is another form, composed of oblique plates set with tessel- 

 lated or oblong teeth, but this may be only a variation of the 

 first named. The mandible is altogether wanting in carni- 

 vorous Pulmonata, or those which merely cut their food in 

 small pieces and swallow it whole ; and in marine molluscs 

 it is found only in a few species. It is seen in the young 

 Limax when quite in the embryo state ; sometimes before it 

 leaves the egg it is observed to be divided into two parts. 

 In addition to the mandible proper, there is, in nearly all 

 the Tsenioglossa, two other lateral plates, or small-sized fixed 

 mandibles, described by Dr. Morch as " cheek-plates,'''' and 

 without cutting edges, " apparently serving only to protect 

 the mouth from injury," or probably serving the purpose of 

 the tongue-bones in vertebrata. Some of the flesh-eaters 

 have the prehensile spiny collar placed quite at the ex- 

 tremity of their proboscis, as in Ancula ; in Nudibranchs it 

 is a formidable weapon. In Cephalopods the mandible 

 should rather be termed maxilla or jaw, for it is fairly divisible 

 into an upper and lower jaw. 



But to return to the teeth of mollusca. These are mostly 



* Some authors — Mr. Jeffreys among the latest — on describing the 

 median part of the band, still apply to it the term rachis. The use of this 

 term is objectionable as applied to anything pertaining to an animal mem- 

 brane. Inasmuch as the word simply means "a spine," and the tongue 

 of the mollusc bears the faintest resemblance to the vertebrate spine, 

 and finding also that the term has been long appropriated by botanical 

 writers, it is uiiadvisable that it should longer be employed when describing 

 tlie median part of the tongue of a soft-bodied animal. 



