1909] Tin-nii : Tin Leptomedusae of l In San Diego Region. 17 



between their marginal sense organs, however, such an associa- 

 te f these genera appears to be rather artificial. Recognizing 



this fact, Maas lms removed Laodice to the Thaumantiidae. He 

 has retained the family Lafoeidae, but redefined it so as to in- 

 clude all Leptomedusae with Large open statocysts, viz., Mitro- 

 coma Haeckel, Tiaropsis Ag., Phialis Haeckel and Halopsis Ag. 



This group of genera appears to be natural, but to refer it to 

 the family Lafoeidae is open to objections. (1) That family is 

 based on a genus of hydroids that produces no free medusae, 

 hence does not possess the characters of the family as defined 

 by Maas. (2) It is not clear that Cuspidella is as closely related 

 to Lafoea as it is to Calycella and Campanulina, members of 

 another family of hydroids; the discovery of its hydroid form, 

 therefore, does not serve to place Mitrocoma definitely among the 

 Lafoeidae. (3) Both Laodice and Mitrocoma produce generic- 

 ally identical hydroids. Yet they are placed in different families 

 by .Maas. as. in my judgment also, they should he. 



In the light of these facts, a new family is needed for the 

 four genera under discussion, with a semis of medusae, not 

 hydroids that lack medusae, as type. 



As such a genus. Mitrocoma appeal's to be unobjectionable, 

 the family name becoming Mitrocomidae. Haeckel 1 1880) used 

 this designation in a schema on p. 163 for all his Eueopidae 

 without gastric peduncles, which artificial group contained Tia- 

 ropsis, Phialis and Mitrocoma. The existing term. then, removed 

 from the Eueopidae with the genera just mentioned, seems, with 

 a new characterization, to satisfy the present requirements. 



Gen. Mitrocoma Haeckel, 1861. 

 Mitrocomidae with numerous open statocysts. numerous ten- 

 tacles and marginal cirri, and four radial canals; no gastric 

 peduncle. 



Mitrocoma discoidea, n. sp. 

 Fig. 4. 

 Umbrella flat, three to four times as broad as high. Manu- 

 brium small and short, with four narrow, ruffled oral lobes. 

 Tentacles 180-240, swollen at the base. Statocysts 20-60. scat- 

 tered, with numerous statoliths in 2-3 rows. .Marginal cirri 



