No. 26.] ARTHROSTRACA OF CONNECTICUT. 21 



forward and usually upward between the bases of the antennae, 

 while that of the maxillipeds extends downward and forward, 

 with the terminal joint bent somewhat toward the middle line. 



The mouth is bounded above by the upper lip, which is a 

 broad plate, circular or oval in outline and continuous by its 

 front margin with the epistome. The epistome forms the ante- 

 rior, ventral surface of the head. The upper lip may form a 

 kind of rostrum projecting forward as in Ericthonius or it may 

 be small and vertical in position as in the Lysianassidse. 



The mandibles bound the mouth laterally and form the prin- 

 cipal chewing Organs, being admirably adapted for tearing and 

 cutting by their strength and heavy calcification. In general 

 the mandibles are triangular in shape with the anterior margins 

 inclined toward the middle line, and, except in the Lysianassidas, 

 bear several strong tooth-like processes. Behind this principal 

 incisive plate, or cutting edge, there is often a secondary one 

 movably articulating with it, which is weaker in structure, but 

 likewise usually denticulated. Posterior to the cutting edges, 

 the mandibles generally bear a molar tubercle having the form 

 of a cylinder or truncated cone, which is directed internally and 

 backward, and is armed distally with rasp-like teeth. The tubercle 

 is absent in the Lysianassidse. There may also be present a 

 palp, consisting of never more than three joints, which is inserted 

 towards the middle of the anterior surface. In the Orchestiidse, 

 Dexamine, Stenothoe, and Caprella, the palp is lacking; in 

 Siphoncecetes it is uniarticulate ; and in Corophium it is made up 

 of two joints. In species having a triarticulate palp the first 

 joint is always short, except in the Hyperiidea, where it may be 

 somewhat elongated, but is never as long as the last two joints. 

 The terminal joint may be somewhat spatulate as in Ericthonius 

 but more generally it tapers to a slender point as in Gammarus 

 locusta and Elasmopus. 



The loiver lip is inserted immediately beneath the mouth, 

 and is made up of two halves which are united on the middle 

 line for a greater or less distance from the posterior end. Each 

 plate has a regular anterior margin of semicircular form except 

 in the genera Amphithoe and Gruhia, in which there is a deep, 

 rounded sinus, making the plate bilobed. The postero-lateral 

 angles are always more or less prolonged to form the maxillary 



