74 Dr. Morch on the Homology of 



The maxilla of the aquatic Pulmonata is provided with a lateral, 

 linear, moveable appendage on each side, the real homology of 

 which is not yet quite ascertained, nor even whether it is a part 

 of the maxilla or independent of it. 



The maxilla is divided into two* in the case of the young Lmaa? 

 before it leaves the egg. The two halves are afterwards united 

 by an intermediate piece, which, in a more advanced period of 

 growth, is indicated by a notch in the projecting middle tooth. 

 I do not believe this circumstance shows that the maxilla is formed 

 by a union of the lateral mandibles. 



A superior and inferior maxillae are only found in the Cephalo- 

 poda, and perhaps in the larval state of some Pectinibranchiata ; 

 but it is not placed beyond doubt that the beak of the Ccphalopod 

 is really homologous with the maxilla in the Pulmonata. The 

 maxilla of Succinea is not unlike the upper maxilla in the Ce- 

 phalopoda. Prof. Van Beneden^s figure of the " machoire supe- 

 rieure et inferieure " of Amphipeplea glutinosa (Exercices Zooto- 

 miques, f . 6-9) looks very like the beak of a Cephalopod ; but 

 it may be erroneous, because the radula is represented (f. 7) as 

 forming a part of the lower maxilla. A similar mistake was 

 committed by Moquin-Tandon, who represented in Neritina as 

 superior and inferior maxillse what, according to Claparede, are 

 only the edges of a corneous membrane lining the oral cavity. 



Messrs. Alder and Hancock have shown, in Acanthodoris 

 pilosa, a solitary inferior maxilla represented by a flat plate with 

 a split in front protruding from the mouth ; but this part seems 

 to me more likely to belong to the lingual cartilages. 



Perhaps the " anterior or lower lip, armed with teeth," repre- 

 sented by Macdonaldf in Clio caudata is the edge of an inferior 

 maxilla. 



The mandibulcB are two lateral concave plates, not unlike a 

 bivalve shell J, the free edge of which (masticatory edge § of 

 Bergh) is either tuberculated or denticulated. From the position 

 of the muscles and from the form of the pectinated edge in 

 Phyllodesmium, Ehrb. (Bergh, 'Anatomic^), it appears that the 

 mandibles cannot be used for mastication, but are only adapted 

 for use as a pair of forceps to hold the food during the triturating 

 action of the radula. By the umbones of the mandibles (umbi- 

 licus, Midd., Bergh) the two halves are united with muscular 

 ligaments. This kind of lateral mandibles is found in the 

 Pleurognatha Gymnobranchiata {JBolidice, Diphyllidice, &c). In 



* Heynemann, Malacozoologische Blatter, 1861, p. 143, The same is 

 the case with Vitrina in its young state. 



t Trans. Royal Soc. Edmb. vol. xxiii. pt. 2. p. 188, t. 9. f. 3. 

 X Described as such by Brown in Binghamia paradoxa. 

 § Cutting edge (Alder and Hancock). 



