NO. 3563 EUPHILOMEDES AROSTRATA—KORNICKER 3 
margin of rostrum and anteroventral margin of shell with scalloped 
outline formed by crescent-like marginal denticulations. Shell sur- 
face with irregular polygons and normal pore canals; polygons con- 
taining pore canals smaller than average; about 23 polygons larger 
than average, hyaline, slightly in front of valve middle, representing 
locations of muscle attachments; hyaline, large, oval spot, with polygon 
border lies dorsal to hyaline polygons (fig. 2F). Anterior two-thirds 
of dorsal margin of valves hinged by transparent ligament following 
arcuate curvature of shell outline; posterior one-third with tripartite 
straight hinge. Hinge not affecting arcuate shell outline, but clearly 
visible in dorsal view and in lateral view when light is passed through 
closed shell. Posterior elements on each valve an angular sclerotized 
process (figs. 24—E); left valve process containing a small forward- 
facing recess which apparently receives backward-projecting process 
of right valve (figs. 2B, c). Thin flap behind posterior process of left 
valve projecting medially, apparently resting on smooth, slightly 
depressed surface behind posterior process of right valve when shell 
is closed; upper surface of flap containing polygons similar to those on 
lateral surface of valves. Anterior hinge element less prominent than 
posterior; no tooth-and-socket arrangement observed. Medial hinge 
element of each valve consisting of straight bar without teeth. Sur- 
face of each valve above medial hinge element bearing two rows of 
minute tubercles or crenulations. Left valve broadly overlapping 
right along anterodorsal margin. 
Thin sclerotized septum projecting downward from dorsal margin 
of each valve, starting near middle in front of anterior hinge element 
and terminating just behind posterior hinge element. Septum clearly 
visible in medial view at anterior and posterior hinge elements where 
it is set back from contact plane of valves; septum striated at anterior 
hinge elements. Several septa seem to be present in vicinity of 
anterior hinge element, especially on left valve. 
Inner lamella (inner lamella, as used here, is peripheral part of 
chitin lining of epidermis) broad with vestibule, terminating at each 
end of dorsal ligament. Line of concrescence lies about halfway 
between inner and outer margin of inner lamella, closer to inner 
margin in anteroventral section, closer to outer margin in postero- 
dorsal section. Marginal zone with simple, fairly closely spaced true 
and false radial pore canals. Selvage narrow, with wide, corrugated, 
lamellar prolongation having serrate outer edge and terminal spines 
(figs. lr, 2c). List midway between selvage and inner margin of 
inner lamella; about ten striations between list and inner margin of 
anterior part of inner lamella (figs. 1G, H). 
Hairs, flaring distally and with concave, cuplike ends abundant, 
protruding from normal pore canals (figs. 24, H); short, tapered hairs 
