EEPORT ON THE GENUS OEBITOLITES. 29 



medial instead of from their superficial ends, seem to be crossed by the septal planes that 

 divide the annular canals.^ The meaning of this appearance is best seen in a transparent 

 section (PI. IV. fig. 8) ; which, traversing the annular galleries, shows the large orifices 

 c, c, c, along their floors, that lead to the chamberlets of the plane beneath ; and the 

 small orifices st, st, along the concave borders of the septa, that lead into the chamberlets 

 of the next annulus. 



Thus the cavitary system of the disk of Orhitolites duplex is composed of a median 

 series of annular concentric galleries, each freely opening, as in Orhitolites margincdis, into 

 a double tier of chamberlets, one above and the other beneath ; but each having two series 

 of passages, that lead severally to the upper and the lower tiers of chamberlets of the next 

 annulus. The cavitary system of every part is here in as free communication with that of 

 every other part as it is in the " simple " forms previously described : but there is an 

 advance in the development of this type, which shows itself Jirst, in the suppression of 

 the " orbiculine " stage, and tlie assumption of the cyclical plan almost from the begin- 

 ning ; and second, in the foreshadowing of the coming separation between the upper and 

 the lower tiers of columnar sub-segments in Orhitolites complanata, which is given in 

 the replacement of the single row of radial stolon-passages on the mesial plane, com- 

 municating between the cavitary system of each annulus and the next, by the douhle 

 series that lead into the chamberlets of its upper and its lower tiers respectively. 



Geographical and Batliymetrical Distrihution.- — This species appears to have the 

 same general range, alike in area and in depth, with Orhitolites marginalis ; but though 

 smaU forms of it are abundant in the Red Sea, I have not been able to trace it as accom- 

 panying that species into the Mediterranean. Like Orhitolites marginalis, it acquires its 

 maximum size in waters of no great depth ofi" tropical shores. 



Geological Distrihution. — Though it is impossible to identify, with any certainty, 

 our Orhitolites duplex with any of the fossil species described by Lamarck, yet as worn 

 specimens of it often present a close resemblance to the representation given by Goldfuss 

 (Petrefacta, PI. XII. fig. 8) of his Orhitolites macropora, which he distinguishes by its 

 poris utroque latere majnscidis, I think it probable that the two are identical. The 

 fossil habitat of Lamarck's specimens is given as "la montagne Sainte Pierre." 



4. Orhitolites complanata, Lamarck (PI. V. figs. 11-18 ; PI. VI., VIL, VIIL). 



Orhitolites complanata, Lamarck, Syst. des Anim. sans Vertebres [1801]. 



Marginopora vertehralis, Quoy and Gaimard, in De Blainville's Manuel de I'Actinologie [1834], 



p. 412. 

 OrUculina tonga, Williamson, Trans. Micr. Soc, vol. iil 1852, p. 115. 



We come, lastly, to the large and highly difi"erentiated form which must be regarded 



1 The relation of fig. 9 (PI. III.) to fig. 11 will be better understood by conceiving the former to be turned a quarter 

 round, so that the side a of fig. 9 shoidd correspond with the lower border of fig. 11. 



