464 BULLETIN" 120, UNITED STATES NATIONAL MUSEUM 



E^. Postero-lateral spines directed obliquely forward, villosum, p. 482. 

 D^. Carapace not unusually sculptured between the epibranchial spines. 



Rostral horns longer and slenderer diplacanthum, p. 478. 



B^. Basal antennal segment armed with 2 or more spines. Orbits elongate- 

 tubular. 



C. Rostral spines separated by a U-shaped sinus eutheca, p. 484. 



C^. Rostral spines separated by a V-shaped sinus. Basal antennal segment 



armed with 2 spines forming an oblique line, the outer spine more or 



less distant from the orbital margin. 



D'. Rostral horns narrow, tapering regularly from base to tip. A long 



spine followed by a short spine near distal margin of basal antennal 



segment laevigatum, p. 483. 



D^. Rostral horns broad at base, with abruptly narrowed tip. Two long 

 spines near*distal margin of basal antennal segment. 

 El. A spine present on the urogastric or anterior cardiac region. Width 

 of carapace across orbital regions much less than across branchial 



regions concavum, p. 487. 



E2. No spine on the urogastric or anterior cardiac region. Width of 

 carapace across orbital regions subequal to width across branchial 



regions.. intermedium, p. 486. 



A2. Carapace with 7 strong spines on its posterior half. 

 B^ Basal antennal segment armed with only one spine. 



C^ Rostrum nearly horizontal, horns long. Basal antennal spine hori- 

 zontal and wholly visible from above camptocerum, p. 469. 



C^. Rostrum stro;igly defiexed, horns short. Basal antennal spine directed 



downward, scarcely visible from above heptacanthum, p. 473. 



B^. Basal antennal segment armed with two spines in a transverse line. 



septemspinosum, p. 477. 



Analogous species on opposite sides of the continent: septem- 

 spinosum (Atlantic), heptacanthum (Pacific); suhparallelum (Atlantic), 

 villosum (Pacific). 



THE TRISPINOSUM-DIC ANTRUM GROUP 



Consists of the so-called species trispinosum and dicanthum. This 

 group of forms is represented in the Museum by so large a number 

 of specimens that we are enabled to separate them into three series, 

 one series being intermediate between the others in at least two^of 

 its characters. In one series (1) (see trispinosum, below), the postero- 

 lateral prominences are narrow, regularly tapering spines, projecting 

 beyond the general outline of the carapace and directed more or less 

 backward, sometimes strongly curved from base to tip (concave 

 above). The carapace is considerably constricted behind the 

 orbits. In the next series (2) (see trispinosum, variety, below), 

 the postero-lateral prominences are wider than in series 1, less spine- 

 like and more laminate, their hind margins nearly transverse; while 

 the carapace is less narrowed behind the orbits. In the third series 

 (3) (see trispinosum nodipes, below) , the postero-lateral prominences 

 are very broad and obtuse, broader than in series 2, their margins 

 almost continuing the direction of the marginal lines of the carapace. 



