JUNE, 1906. New Forms or CONCRETIONS—-NICHOLS. 43 
thickly covered, and partially obliterated by other incrustations. 
Other portions are covered with somewhat smaller club-shaped branch- 
ing forms of bryozoa. The entire surfaces of all the specimens are 
covered with films of encrusting bryozoa, and of a nullipore allied to 
Meloboesta of which many were living when the nodules were col- 
lected. The surfaces show also a multitude of forms of other 
calcareous organisms, including curved worm tubes, fan-like forms, 
etc., of occasional occurrence. The specimens collected in 1873 
by the Challenger were covered with lving Madracts, which 
appear in the present specimens to have been replaced for the most 
part by nullipora and by bryozoa of encrusting rather than branching 
forms. The larger branching corals, etc., are confined to one-half of 
the surface, the other half being fairly smooth, and coated only with 
the smoother encrusting forms. This smooth half probably is the 
part embedded in the calcareous ooze from which the nodules were 
dredged. To some specimens are attached completely encrusted 
shells of sizes up to 45 millimeters. (Plate XXVI, Fig. 4.) The 
nodules are penetrated frequently by syphon tubes of a Pholas-like 
shell, These shells were all dead when collected and filled with 
calcareous sand. Some of the boring mussels are also represented by 
long-dead shells. There are also numerous serpula-like calcareous 
tubes penetrating the nodules in every direction. The calcite of the 
surface is of a friable, chalky, and earthy character, giving no 
indications of macroscopic crystallization. For purpose of study 
several specimens were sawn through the centre with a_hack- 
saw. These sections (Plate X XVII) exhibit a chalky, cellular lime- 
stone, becoming more solid and denser toward the centre. The cells 
possess no regularity in form, size, or distribution. Some of the 
openings are sections of the syphons of Pholas or some allied form, of 
worm tubes and of pelecypod shells; more are merely irregular 
cavities in the limestone. Towards the centre the cells are smaller, 
with thicker walls, and toward the surface they are larger, with 
thinner walls. Upon examination from a distance, the cells have a 
distinctly concentric arrangement, which disappears upon close 
examination, except near the surface. Close to the surface, and fora 
distance inward of six or eight millimeters, the material is in the form 
of concentric, irregularly waved sheets of calcite, which touch ‘and 
coalesce in spots enclosing elongated, empty cells lying approximately 
parallel with the surface. Upon the outside of the nodules there are, 
in places, thin encrusting bryozoa and algae, which arch away from the 
nodule in a similar manner. This type of cellular structure dies out 
