548 Transactions of the Society. 



internal structures, and I have omitted these details from all 

 drawings, as they would only obscure the structure of the lorica, 

 in which all specific distinctions are found. 



The lorica, into which the head can be completely retracted, 

 consists of a dorsal and ventral plate, connected by a flexible 

 membrane, and a posterior prolongation usually supposed to be 

 part of the ventral plate, but which I believe to be really the 

 dorsal plate of a second segment. The lateral view (see figs. 2h 

 and 13^) always confirms this theory of the relation of parts. 



The anterior margin of the lorica, which can only be seen when 

 the head is retracted, affords one of the best and most constant 

 specific characters, and therefore for systematic purposes the 

 animals are always represented fully contracted. 



Cathifpna lofuana (fig. 1) is the most suitable figure to illustrate 

 the structure of the lorica. The dorsal plate, which in this species 

 is narrower than the ventral, is indicated by a darker line. Its 

 posterior border is almost invariably a very flat convex, rarely 

 more rounded (figs. 6, 8, 9). The lateral margin is sharply 

 defined till it approaches the anterior margin. Sometimes it can 

 be traced clearly to the anterior margin (figs. 3, 15, 17, etc.), but 

 often, as in (7. lofuana^ it is bent or curved inwards and disappears 

 before reaching the anterior margin. It is a problem I offer to 

 students to determine the anterior limits of the dorsal plate in such 

 cases. 



The ventral plate terminates anteriorly in angles, rarely 

 rounded off (figs. 11, 16, etc.), often produced as little spines 



EXPLANATION OF PLATE XXIII. 



Fig. 12a. — Gathyiyna stokesii. English form ; dorsal view. 



,, 125. Ditto. English form; ventral view. 



,, 12c. Ditto. American form ; dorsal view. 



,, 12d. Ditto. Lateral view. 



„ 12e. Ditto. Lateral view of toes. 



,, 13a. — C. signifera. Dorsal view. 



,, 136. Ditto. Lateral view. 



,, 14a. — C. ohioensis. Dorsal view. 



„ 146. Ditto. Toes. 



,, 15. — C. brachydactyla. Dorsal view. 



,, 16. — C. ligona. 



,, 17. — C. spinifera. 



„ 18. — C. deiyres^a. ' 



,, 19a. — C. muscicola. Dorsal view. 



,, 196. Ditto. Ventral view. 



„ 19c. Ditto. Toes. 



,, 20a. — Monostyla sp. (?) 



,, 206. Ditto. Front of lorica more enlarged. 



,, 21a. — M. rohusta. Dorsal view. 



„ 216.— Ditto. Toe. 



(All figures magnified 260 diam.) 



