Ivi 
SYNOPSIS OF GENERA. 
Diagnosis. Carapace bivalvular, sub-quadrate or sub-ovate in 
outline, posterior margin transverse or incurved. Hinge-line 
straight, nearly equaling the length of the valves. Valves 
slightly gaping at the anterior extremity. Rostrum narrow 
and elongate, ornamented with longitudinal ridges. Cephalic 
region characterized by low, rounded, indistinct elevations. 
Optic node well defined. Surface with one or more strong 
longitudinal carinae. Abdomen, as far as known, composed of 
two segments, which are sub-cylindrical and without nodes or 
spines. 
Family, Pinacarid^e. 
Genus MESOTHYRA, nov. gen. 
(See Plate xxxii.) 
Diagnosis. Carapace sub-quadrate in outline, composed of two valves which 
come into contact at the apices of two broad, sub-triangular extensions, situated 
on the dorsal line opposite the eye-nodes, forming a short and broad anterior, 
or rostral cleft and a long posterior cleft. Rostrum not observed. The pos¬ 
terior dorsal cleft was covered either by a single elongate plate, with each 
margin of which the valves were in symphysis, or, as is more probable, by a 
double plate, divided by a median suture. Test broadly infolded on the lower 
surface, thickened and produced into a conspicuous and acute posterior spine. 
Posterior margin incurved and produced into a short spine at the dorsal line. 
Surface with a single strong carina on each valve. Abdomen consisting of two 
somites of which the posterior is the longer. Post-abdomen with a broad caudal 
plate, which is produced into a relatively short telson. Lateral spines long 
and setaceous. 
This genus is established to include certain species hitherto referred, on in¬ 
sufficient grounds, to the genus Dithyrocaris, Scouler. The peculiar structure of 
the hinge area separates it widely from Dithyrocaris, unless the type species of 
that genus has been inadequately described. There is, however, so close a 
similarity in these forms of Mesothyra with many of the described species of 
Dithyrocaris, in all respects except the feature noticed, as to give rise to a sus- 
