756 THE AMERICAN NATURALIST. [VoL. XXXV. 
cro. Head surmounted by an arched chitinous shield, appearing in 
side view like a hook; lorica either arched, and compressed 
laterally, or dorso-ventrally flattene 
dr. Lorica arched, somewhat compressed laterally, so as to be 
higher than wide; open behind and in front, and sometimes 
open on the ventral side. Minute, inconspicuous rotifers 
Colurus 
ez. Lorica ovate as viewed from above ; produced caudally 
(as seen in dorsal view) into two acute spines, separated by a 
deep sinus ; ventral surface of lorica cleft 
224, C. deffexus Ehr. (Fig. 158) 
e2. Lorica not cleft ventrally ; excavate behind, so as to form 
two teeth with a shallow notch between them 
225, C. bicuspidatus Ehr. (Fig. 159) 
£3. Pear-shaped, widest behind, in dorsal view ; ventral sur- 
face cleft; caudal notch very shallow, between two short 
terminal points ; foot and toes three-fourths the length of the 
MODA Secu . . 226, C.caudatus Ehr. 
e4. Ovate in ion. or dein view, the posterior end rounded, 
without points ; lorica cleft ventrally. Foot very small 
227, C. obtusus Gosse (Fig. 160) 
£5. Elongate-ovate in dorsal view ; caudal margin rounded, 
without cleft or points ; ventral cleft interrupted in front of 
the middle, so that the openings for the head and foot are 
not continuous. . 228, C. agilis Stokes (Fig. 161) 
eó. Lorica ovate in decl view, high behind, low in front; a 
long, stiff, pointed, hyaline crest extending backward and . 
upward from the "re of the anterior dorsal edge of the 
lorica  . , C. cristatus Rousselet (Fig. 162) 
d2. Height and width of Vitas about the same ; lorica forming 
an ovate box with surface marked into areas ; a thin ridge on 
the mid-dorsal line ; a similar ridge on the mid-ventral line from 
the anterior margin to pag the middle of the length 
o, Metopidia salpina Ehr. (Fig. 163) 
d3. Lorica flattened, bas than high, usually turtle-like in 
appearance; open only in front and behind, not along the 
ventral middle line ... Metopidia 
ez. Outline of lorica varying tom elliptical through oval and 
ovate to orbicular, but without teeth, spines, or prominent 
angles (except at the sides of the foot and head, where angles 
are necessarily formed, owing to the openings in the lorica) 
ft. Lorica nearly circular, much depressed ; dorsal plate 
with a very low decia median ridge and with a submar- 
ginal line of corrugati . . 231, M. solidus Gosse 
Je. Lorca oval or ges ph depressed, evenly rounded 
