63 
Echinoids“ Pis. 2—3) x ), and this was the specimen with the two 
kinds of large globiferous pedicellariæ, as I was informed by Pro¬ 
fessor Clark. On account of this I suggested (in „Die Echinoiden 
der Deutschen Siidpolar Expedition“. 1909, p. 47) that this spe¬ 
cimen represented a hybrid between Tretocidaris Bartletti and 
Stylocidaris afjinis. Having later on had occasion to examine a 
specimen with the same form of spines and the two kinds of 
globiferous pedicellariæ I feel more convinced that this is really 
a hybrid between the two species named („On soine West Indian 
Echini“ p. 10). — This case is the more interesting, as it repre¬ 
sents a hybrid between two distinet genera. The characters of the 
hybrid are mainly like those of Tretocidaris Bartletti ; as characters 
inherited from the other supposed parent species, Stylocidaris 
afjinis , must be regarded the cylindrical, nearly smooth spines and 
the one kind of large globiferous pedicellariæ (without endtooth), 
this form being (in the specimen examined by me) mueh rarer 
than that characteristic of Tretocidaris (with a long endtooth and 
a very small pore). 
In the „Echinoidea of the Swedish Southpolar Expedition u 2 ), 
p. 48, I have mentioned a specimen, which I suppose to be a 
hybrid between Sterechinus Agassizii Mrtsn. and St. Neumayeri 
(Meissner). It affords a remarkable confusion of the characters 
of these two species; but as the species of the genus Sterechinus 
do not, upon the whole, differ very conspicuously from each other, 
this case of hybridisation is not very conspicuous either. 
To the above recorded instances of hybrids of Echinoids I can 
now add a new one, and even one of unusual interest, being a 
hybrid between two different genera and one, of which there can 
scarcely be any doubt. that it really represents a case of hybridisation. 
On an excursion to the Sound, North of the Island of Hven, 
with the steamer of the Danish Biological Station „Sallingsund“, 
the lOtli September this year (1910), I collected a specimen of 
9 Buli. U. S. Nat. Museum. 74. 1910. 
2 ) Schwedische Siidpolar Exped. 1901—1903. Bd. VI. 4. 1910. 
