THE EXO-SKELETON OE THE PROBOSCIS. 143 



oral lobes. Gerstfeldt and Menzbier follow Kirby and Spence, 

 and term the thyroid the mentum. 



The Furca (Fig. 28, 5) is situated on the back of the oral 

 sucker ; it presents a triangular plate, the angles of which are 

 processes prolonged to the edges of the disc. The posterior 

 process is short and slender, the lateral processes are very 

 strong, and each exhibits an elongated fossa near its origin, 

 which articulates with the corresponding cornu of the thyroid. 



When the oral sucker is closed, the lateral processes of 

 the furca are parallel with each other, and appear to be mere 

 continuations of the thyroid cornua. When the disc is 

 expanded these processes lie in the same plane, or are even 

 bent back at an obtuse angle by the action of a pair of power- 

 ful muscles inserted into them. When the disc is expanded 

 the thyroid cornua are bent back and slide outwards in the 

 grooves in the furca. The closure of the disc is due to the 

 elasticity of the thyroid cornua, which act on the furca like a 

 pair of springs. 



The Epifurca. — I apply this term to a slender rod of chitin 

 (PI. VI., cp f), near the margin of the posterior surface of the 

 disc, the axis of which is at right angles with the lateral pro- 

 cess of the furca. 



The Hypoglossa or stomal plate (Fig. 28, 4, st) is a thin, 

 gutter-like plate which forms the floor of the groove in the 

 theca. It does not extend as far back as the base of the ligula, 

 and terminates at its distal end in a thin sharp-pointed edge, 

 which projects between the lateral lobes of the stomal disc. 



The Paraphyses (PI. VI., pa), are regarded by Kraepelin as the 

 thickened edges of the plate I have termed the hypoglossa. In 

 the young imago they are distinct rod-like sclerites. The 

 distal ends of the paraphyses exhibit articular surfaces, which 

 form a kind of hinge-joint with the discal sclerites. 



The Discal Sclerites (Fig. 28, 4, ch) lie deeply in the cleft between 

 the two lateral halves of the oral disc. I shall term this cleft 

 the prcstomuin. The discal sclerites form the upper chitinous 

 fork of Kraepelin. 



Each discal sclerite consists of a quadrilateral elongated 



