794 The American Naturalist. [September, 
species) consists of four distinct and separate parts; two broad 
lateral plates or valves, a narrow median or axial plate, and 
an anterior median plate or rostrum. 
The valves have an elongate, somewhatsemi-oval marginal out- 
line, narrower posteriorly than anteriorly, with a truncate or 
concave posterior extremity. They come into contact at only a 
single point, and that is in the axial line at about one-fourth 
of the median length from the anterior margin. At this point 
a projecting angle is formed on each by the attenuation and 
termination of the rostrum and median plate, and the project- 
ing points appear to come into simple apposition, though it is 
possible that they slightly overlap; there is, however, no satis- 
factory evidence of the clasping of the valves at this point, 
such as that occurring in the great species Mesothyra Oceani2 
Fic, 1. Diagrz ic fi howing the carapace structure in Rhinocaris. 
Opposite this point of contact and considerably within the 
median line of each valve, is a visual node having the form 
* Op. cit. pl. xxxii, fig. 6; pl. xxxiii, figs. 4, 5. 
