538 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Musculature of mantle very well developed; composed of rather 
thick closely placed bands. Those which radiate from the siphons 
extend to about the middle on the sides of the body, and along the 
endostyle to a still greater distance. The circular muscles of the 
siphons underlie these bands. On all parts of the median line except 
Text-f5g. 26. — Microcosmus nacreus, sp. nov. Slightly enlarged. 
where the siphons arise, stout closely placed transverse bands, extend- 
ing well on to the sides of the body, overlie the above mentioned 
radiating bands. 
There are eight large, twice (to a small extent three times) compound 
tentacles, irregularly arranged. A considerable number of smaller 
tentacles of different sizes in the intervals but it is difficult to recognize 
distinct orders or a regular scheme of arrangement. Tentacles pro- 
vided with moderately wide membranes. Their branching is of the 
pinnate type with short lateral branches. Tips of branchlets not 
enlarged. 
Dorsal tubercle horseshoe-shaped; open inter\'al directed anteriorly; 
horns incurved. 
Dorsal lamina a membrane whose edge has numerous low obtusely 
triangular denticulations. 
Branchial sac with five well developed folds on each side of the body. 
The second and third folds are highest, though the number of internal 
longitudinal vessels borne on the folds decreases from the first to the 
fifth. Folds higher in anterior part of sac. Details of structure of 
sac practically alike on both sides. Transverse vessels of four orders 
(the larger ones bearing narrow membranes) cross the sac and are 
quite regular in their arrangement according to the scheme 1, 4, 3, 4, 
