530 Mr. D. Sharp and Mr. F. Muir on the Comparative 
Family BYRRHIDAE. 
Form examined: Byrrhus gigas Fabr., Alps. 
B. gigas. 
Of the simple trilobe type. Median lobe chitinous along the 
dorsal aspect, the apex of which is cleft, each point flattened and 
slightly twisted; struts at base very short; median orifice 
supported by a very attenuated chitinous ring, the ventral 
margin prolonged into a sharp-pointed lobe, supported on the ventral 
face by a narrow chitin plate. Lateral lobes well developed, their 
bases meeting on dorsal and ventral aspects, and thus enveloping the 
median lobe, their apices pointed ; point of articulation on the dorsal 
aspect. Basal-piece triangular. Internal sac undifferentiated. 
This is very like Hydrophilus, 
Family CHELONARIIDAE. 
Form examined: Chelonariwm zapotense Sharp, Guate- 
mala, and C. errans Sh. 
Chelonarium zapotense (Pl. LXII figs. 122, 122a). 
This is a highly specialised trilobe form. Median lobe short, stout, 
highly chitinised; the ventral side being drawn out into two long 
median struts. To the ends of these struts is articulated a median 
process, bilobed at the base, slender in the middle and divided into 
two long, slender, flat, bent, distal processes (a). Lateral lobes small, 
rounded at apex and curved. Basal-piece formed by a large sclerite 
on the ventral aspect, with lateral and basal edges curved up; 
dorsal side membranous. Internal sac undifferentiated. 
C. errans is exactly the same type, but some details are different 
(i. e. the slender process (a) from the ventral aspect is single). 
Family CYATHOCERIDAE. 
Form examined : Cyathocerus horni Sh., Central America, 
Figs. 123 and 128a Pl. LXII. 
C. horni (Pl. XLII figs. 123, 1232). 
Median lobe long, thin and crooked, with median orifice at apex 
and median foramen at base on dorsal aspect. Tegmen forming 
sheath with the distal end cleft along the dorsal aspect, but with- 
out division between lateral lobes and basal-piece. Internal sac 
undifferentiated. 
