234 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
present species have their front edges in a nearly straight line, slightly notched 
medially, and extend about four times the width of the shell, or some 20 mm. 
When folded preparatory to complete retraction, they are contracted about one- 
fourth of their extent. 
The parapodia are roughly triangular with the anterior margin longest, the 
lateral margin distinetly trilobate and passing imperceptibly into the posterior 
lamina, which is slightly reflected over the reflected anterior edge of the ventral 
plate. In no ease was this lamina seen to extend over the convexity of the ven- 
tral plate, as it is asserted to do in the Mediterranean species. If the dimensions 
of the figures of the animal in the plates of the ** Bonite " are taken from the fully 
expanded animal (whieh I strongly doubt), they are hardly half as large as those 
of our Pacific variety. But the contractibility of these delicate organs is so 
excessive that in a dead or exhausted specimen the real extent of the fins is never 
shown, much less in one which has been subjected to a preservative fluid. The 
appendages of the mantle which extend from the lateral slits are short and also 
trilobed, but not deeply; in the Mediterranean form they are represented as 
entire or obscurely bilobate, and longer. he color of the living animal is trans- 
lucent yellowish, with two large, sharply defined dark violet spots on the para- 
podia, and a paler violet of some of the viscera shining through the shell. In the 
Mediterranean form the violet is represented as gradually fading out toward the | 
edge of the parapodia, but in the Pacific variety the violet area is dark up to its 
extreme edges and not graduated in any way. The anterior edge is mesially 
indented; behind and above it extend two small tentacles (figure 1e) minutely 
swollen at their distal ends. "The right-hand one is about twice as long as the 
left one. The mouth is axial in its longest diameter, narrow, with raised margin 
slightly pointed in front. 
In captivity in a bowl of fresh sea water the individuals kept up swimming for 
some time. When exhausted, they drew in the parapodia and slowly sank to the 
bottom of the bowl. After a rest they would resume their activity, and several 
of them lived for three days, the water being frequently renewed. 
The most nearly related species, so far as the soft parts are concerned, to our 
Pacific variety is C. gibbosa Rang, as figured by D'Orbigny in the Voyage dans 
l'Amerique Méridionale, but this species has a shell of quite different form. 
Hyalaea truncata Krauss (1848), which appears to be the same as M. cumingi 
Deshayes (1877), seems to me from authentic specimens to be a distinct species. 
It is reported from the South Atlantic in latitude 409 S., the Cape of Good Hope, 
and the Indian Ocean. 
The very great discrepancy in size of extreme mutations of the species of Cavo- 
lina has led to the separation of some of them as distinct species, but we have in 
such genera as Cypraea and Marginella other instances of such discrepancies be- 
tween individuals of the same species. Individuals of a single swarm are usually 
nearly identical in size, according to my observations. It is therefore not improb- 
able that the discrepancies observed are due to some conditions of food supply or 
temperature which have affected a particular swarm during its period of adoles- 
cence, which is presumably very short. 
