lvi 



SYNOPSIS OF GENERA. 



Di ignosis. 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 carinoe. Abdomen, as far as known, composed of 

 two segments, which are sub-cylindrical and without nodes or 

 spin 



Family, Pinacarid^:. 



Genus MESOTHYRA, nov. gen. 



(See Plate xxxii.) 



Diagnosis. Carapace sub-quadrate in outline, composed of two valves which 

 conic 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, Scolder. 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 Mesolhyra with many of the described species of 

 Dithyrocaris, in all respects except the feature noticed, as to give rise to a sus- 



