506 PROCEEDINGS; BOSTON SOCIETY NATURAL HISTORY. 
aperture. The specimens appear to have been attached only to loose 
sand and gra^•el, and their exterior, which is rough and irregular, is 
coated with a thick layer of firmly attached coarse sand and has some 
rather large pebbles firmly imbedded in it. The test is rather thick 
and firm and its inner surface is whitish. 
Text-fig. 16. — - Caesira robusta, sp. nov. X 1-8. 
Mantle musculature weak for such large specimens. Some stout 
bands radiate from the base of the siphons and are underlaid by less 
regular circular bands. On the sides of the body the muscles are 
slender, inconspicuous, and irregularly disposed; as the region of the 
endostyle is approached short stout muscle bands become numerous 
but are irregular in their arrangement, lying in various oblique direc- 
tions and crossing each other irregularly. 
Tentacles mostly of three sizes or orders, six of the first, six of the 
second, and twelve of the third order arranged in the usual way (1, 3, 2, 
3, 1, 3, 2, 3, etc.). Some smaller tentacles are present in the intervals 
but they are few and irregularly distributed. The largest tentacles 
are three times compound in a rather more regularly pinnate manner 
than usual in this genus, their branches and branchlets are not very 
numerous, and the tips of the branchlets are moderately tapered. 
Dorsal lamina broad with long, narrow, sharply pointed teeth on the 
posterior part. Some of the internal longitudinal vessels (one or two 
on a fold) bear on their extreme posterior part a few similar teeth. 
(See PI. 51, fig. 45.) 
Dorsal tubercle C-shaped in both specimens (in one rather irregu- 
larly so) with the horns directed to the right and spirally incurved. 
Branchial sac with six well developed folds on each side. In the 
smaller of the two specimens the distribution of the internal longitufli- 
nal vessels is about as follows: 
mdv. (11) (10) (10) (S) (7) (5) en. 
