GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 1523 



abundant in the extra-tropical regions, but besides, the individuals of 

 species appear to be more numerous, or at least not less so. At 

 Fuegia, the quantity of Gammaridaa collected on bait dropped in the 

 water was exceedingly large; and in no region visited by us, did we 

 find evidence of as great profusion. The Spheromse were also very 

 abundant along the shores. 



VI. Moreover, the species of extra-tropical waters are the largest of 

 the tribe. In the Frigid zone, there are Idotasidse three to four inches 

 long, while the average size of the tropical species is less than three- 

 fourths of an inch ; there are Spheromse an inch long, while those of 

 the tropics seldom exceed a fourth of an inch ; there is a Lysianassa 

 three inches long, while the warmer seas afford only small species, 

 half an inch in length; there is a Pterelas over an inch in length, 

 while the iEgidse of the tropics are less than half an inch. The 

 Gammari of the tropics are small slender species, not half the size of 

 those of the colder seas. The species of Serolis are an inch to two 

 inches, long. Thus, through the Idotaeiclea, the iEgidae, Serolidaa, 

 Spheromidae, Caprellidea, and Gammaridea, the largest species belong 

 to the colder seas, and the giants among Tetradecapods, are actually 

 found in the Frigid zone. 



Among the Hyperidea there is one gigantic species, belonging to 

 the genus Cystisoma, which is over three inches long. It is reported 

 from the Indian Ocean, but whether tropical or not is unascertained. 

 Of the species of this group examined by the writer, the largest, a 

 Tauria, was from the Frigid zone. 



VII. Again, the Tetradecapoda of extra-tropical waters are the 

 highest in rank. Among the Isopoda (which stand first), the Idotaei- 

 dea appear to be of superior grade, and these, as observed, are espe- 

 cially developed in the colder seas, reaching their maximum size in 

 the Frigid zone. Again the Serolidaa, the highest of the Anisopoda, 

 are cold-water species. The Orchestiae among the Amphipoda, al- 

 though reaching through both the Torrid and Temperate zones, are 

 largest and much the most numerous in the latter. 



VIII. Those species of a genus that occur in the colder waters, are 

 often more firmly put together, and bear marks of superiority in their 

 habits. The Amphithoe and Gammari of the tropics are lax and 

 slender species, of small size compared with those of the colder seas. 



IX. There is a tendency in the colder waters to the development of 

 spinous species. This fact is as true of the Podophthalmia as of the 



