102 CALIFORNIA ACADEMY OF SCIENCES. 



Key to the Subdivisions of the Macroura. 



Abdomen asymmetrical Paguridea. 



Abdomen symmetrical, 



Exoskeletou calcareous; pereopods mostly six-jointed. 



None of the three anterior pairs of pereopods chelate.. . .Loricata. 

 All of the three anterior pairs of pereopods chelate. . . Homaridea. 

 First pair, or first two pairs of pereopods chelate; third 

 pair never chelate. 

 Last pair of pereopods reduced, slender, folded. 



Thoracic sternum broad Galatheidea. 



Thoracic sternum linear Hippidea. 



Last pair of pereopods not greatly reduced T'halassinidea. 



Exoskeleton generally corneous; pereopods seven- jointed. 

 Antennal scale large. Swimming forms with generally 

 laterally compressed bodies. 



Third pair of pereopods chelate Penceidea. 



Third pair of pereopods not chelate Caridea. 



Subtribe HIPPIDEA. 



Carapace ovate or subquadrate, comparatively smooth, the regions not 

 well defined. Interantennulary region broad. Eyes small. Antennules 

 generallj'^ well developed, with one flagellum elongate, the other short or 

 absent. Antennae with a large five-jointed peduncle, with or without an 

 acicle. Maxillipeds more or less operculiform. Dactyls of the ambula- 

 tory legs flattened; fifth pair of legs slender and flliform. Thoracic 

 sternum linear. Abdomen partially extended; telson large; penultimate 

 abdominal joint with a pair of biramous lamellate appendages, which, 

 however, do not form a caudal tin in connection with the telson. The 

 males have no abdominal appendages, except the last pair. Pterygosto- 

 mian regions free from the epistome. 



Family HIPPID^. 



Antennal acicle small or absent. Maxillipeds operculiform, the merus 

 broad; exognath wanting. First pair of legs not subchelate. Telson 

 elongated, lanceolate. 



Genus Hippa Fabr. 



Antennules of moderate length. Antennal flagellum very long, stout, 

 multiarticulate and strongly ciliated, Maxillipeds with the ischium very 

 small and the merus very large; the terminal joint narrow and compressed. 

 Dactyls of the first pair of legs oval and lamellate. 



Type. — H . emerita L. 



