508 Mr. D. Sharp and Mr. F. Muir on the Comparative 
“ring-piece,” the cap (a) or lateral lobes forming a wide curved plate 
slightly emarginate; the basal-piece forming a large shield-shaped 
plate with a deep keel down the centre (b). Internal sac large, with 
complex armature. 
Corylophus cassidioides is of the same type. At present 
we cannot directly connect this to any other type; the 
small median foramen with the internal sac contained in 
the median lobe is unique among the “ring” forms, where 
it is the rule to have a large median foramen and the 
internal sac passing through it, when not evaginated. 
Family SCYDMAENIDAE. 
Forms examined: Stenichnus collaris Miill., England. 
Humicrus (recently Scydmaenus) tarsatus Miill., England. 
Leptomastax coquerelt Fairm., Corfu. 
Figs. 56, 56a, 6 and ¢, 57 Pl. LI. 
Stenichnus collaris (Pl. LI figs. 56, 56a, 56, 56ce). 
The distal portion of the median lobe forms a short thick irregular 
tube ; the basal part being curved under and prolonged into a flattened 
narrow process (f), a band of membrane (m) connecting the two 
portions ; the median orifice is large, at the distal end ; the median 
foramen small, situate on the dorsal face about two-thirds down the 
tubular distal end of the median lobe. Lateral lobes narrow flat pro- 
cesses, attached to the median lobe at the dorsal edge of the median 
foramen. Internal sac short but very complex (56c). On the dorsal 
face there is a membranous surface bearing a pair of keels studded 
with chitinous teeth (q) which converge together in the centre above 
the opening of the ejaculatory duct ; on the ventral half is a broad 
chitinous plate somewhat shoe-shaped in lateral view (a and )), 
bearing a pair of small toothed processes (h). 
We would like to call attention to the great importance 
of recognising the mobility of the mternal sac and con- 
comitantly the variation in the position of the sac armature, 
especially when it closes the median orifice. Unless this 
is understood the shape of the aedeagus will appear to vary 
greatly in certain species. In the figures we give, fig. 56 
shows a side view with sac invaginated, 56) shows the sac 
partly evaginated, and 56c with it entirely evaginated, or 
nearly so; 56a gives a ventral view of 560. 
