472 University of CaUfornia Puhlications in Zoology [Vol.16 



very numerous iu lowermost stratum of colony, zooids extending into 

 both upper and lower limy strata. Bladder cells not conspicuous but 

 present around closely packed zooids. Zooids very similar to those of 

 Didemnum carnulentum but smaller. The atrial oritice proportion- 

 ately much larger, often over half the length of branchial sac. Testis 

 almost spherical when fully ripe and half as large as sac. Coil of 

 vas deferens contains seven turns. Ripe ova gigantic, even larger pro- 

 portionately than in carnulentum. Budding similar to that of Tridi- 

 demnuni della vallei. Breeding season in June. 



Habitat and distribution. — On under sides of rocks in the littoral 

 zone and in the holdfasts of kelp, hence from depths of a few fathoms. 

 Common at La Jolla and in the San Diego region. 



The variety or sub-species here recognized differs from the typical 

 species in color chiefly, it being opaque white. We do not, however, 

 depend on this difference exclusively for recognizing a group that 

 deserves a separate name. The other differences that seem significant 

 are in the character of the atrial orifice, this being considerably larger 

 in lacteolum; and in the spicules, these being blunter in lacteolum. 

 Possibly significant, too. is the fact that what we are regarding as a 

 variety lives typically in the great masses of kelp holdfasts, at a 

 depth, consequently, of several fathoms. 



The species and variety of Didemnum here described have much in 

 common with several previously described species ; but of their dis- 

 tinctness from all those that have been treated with sufficient detail 

 to enable us to judge, there seems little doubt. 



It appears almost incredible that the one hundred species, more or 

 less, of Didemnum now recognized, can all be really distinct. Yet 

 we cannot assume that specimens from a geographical region as remote 

 as ours from other regions which have furnished the known species, 

 are the same as any that have been named but not described with 

 sufficient definiteness to enable us to make sure of the identity of ours 

 with these. There seems no course open, therefor, but to add still 

 another to the list of supposed species. 



A thorough revision of thi.s group of ascidians based on ample 

 material of all recognized species is much to be desired. 



The milk-whiteness of the colonies suggested the name.. 



Trididemnum della vallei, n. sp. 



PI. 44, figs. 54 to 56 



Superficial characteristics of the colony. — Encrusting on the under 

 surfaces of rocks, colonies being half a foot or more in expanse in some 

 cases ; thickness from 1 to 2 mm. Color gray with tinge of yellow. 



