June, 1906. New Forms of 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 

 Meloboesia 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 living Madracis, 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 XXVII) 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 for a 

 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 



