MUSEUM OF COMPARATIVE ZOOLOGY. 13 
leave a central space free; in consequence of the séales being obtusely rounded | 
at their free angle, the naked central space of the mouth has an almost stellate 
shape; and in each angle of this space, which alternates with the free obtuse 
ends of the scales, a tooth-shaped free pointed end of an underlying scale be- 
comes visible. These “teeth” are easily distinguishable in the smallest, as 
well as largest specimens. The anus is surrounded by small overlapping scales 
in such a manner that no true valves become visible. The scales are covered 
with minute rounded granules. The number of scales between the mouth and 
anus varies; in the largest specimen they are about ten. In all specimens the 
pedicels form a double row round the sole, those in the exterior row piercing 
the margin of the body. 
One of the largest specimens from Barbados has the following measure- 
ments: length 38 mm., breadth 32 mm., and height 25 mm. Consequently, 
it is not depressed, but almost hemispheric, and is covered with numerous 
grains placed upon the scales on the dorsal surface. These grains resemble 
very complicated tables, and consist of a concave perforated disk supporting 
an irregular elongate network with numerous small teeth in the free end. In 
addition to the grains, the exterior layer of the dorsal integument contains 
small, concave perforated cups, which carry numerous obtuse spines in the 
rim. I have not been able to observe such cups in the other specimens at my 
disposal, but they probably have been destroyed, together with the exterior 
layer of the integument. The sole of the above-mentioned large individual 
bears deposits which are often larger and more irregular than is the case in 
the typical specimen. ‘ 
Among the small typical specimens obtained at Sand Key, three are remark- 
able in having anal and oral valves arranged just as in Psolus tuberculosus, and 
in possessing only about three scales between the oral and anal valves. It 
seems very probable that these specimens belong to another species. 
Habitat. Sand Key; depth 110 to 150 fathoms; numerous typical speci- 
mens. Barbados (1878-79); depth 82 to 103 fathoms; three small specimens, 
about 15 mm. long, and one larger specimen, 38 mm. long and 32 mm. broad. 
Psolus tuberculosus, n. sp. 
Figure 5. 
As is seen from the figures, the exterior appearance of this species is very char- 
acteristic. The specimens dredged at Sand Key are the largest, and may be 
considered as types. They measure 30 mm. in length, 16 mm. in breadth, and 
14mm. in height. When the animals are fully extended, these measurements 
become slightly different. The mouth is closed by five large triangular valves, 
which form together a very regular pentagonal shield ; the anus is also closed 
by five small valves, which have the free angle rounded, and which form to- 
gether a small, more rounded pentagon, or anal shield. The dorsal surface is 
very hard and rough from large scales, which appear to he placed side by side 
