138 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



involutions of the shell as the posterior face of the hood. It has been suggested that the black 

 layer seen upon the lower portion of the involution of the shell is deposited by the dorsal portion 

 of the mantle. The epithelium of the posterior surface of the hood is pigmented and glandular, 

 and probably plays the principal part in the deposition of the black material. 



The mantle is for the most part a very thin, almost membranous, fold and is only slightly 

 muscular. The contrast between it and the mantle of most of the Dibranchiata is very striking 

 in this respect. The anterior border of the part of the mantle which is attached to the edge of the 

 shell is slightly thickened and is comparatively quite muscular, forming a band along the edge 

 of the mantle 1 centimeter to 1.5 centimeters in breadth. The median ventral portion of the 

 mantle is also frequently especially muscular, and forms a strip which joins the muscular border 

 like the stem of a T. The edge of the mantle is marked by two parallel grooves separated by a 

 sharp ridge. The mantle seems to be attached to the edge of the shell, not only by its own edge, 

 but also along a narrow band extending back from the edge on the outer side, along which the 

 epithelium is peculiarly modified. Probably this attachment is not very strong. 



The dorsal portion of the mantle is of uniform thickness, and, while very thin, is still much 

 more muscular than the ventral portion. 



The ventral portion of the mantle frequently presents a peculiar appearance. The tissues 

 on either side of the middle line seem to have become chitinized; the mantle in these spots appears 

 thin, transparent, and structureless, and has the appearance of a thin sheet of ehitin. Sometimes 

 the chitinized areas (if we may so call them) are small, like oval windows set in the sides of the 

 mantle; sometimes they extend over the greater part of the ventral portion of the mantle, and 

 sometimes even across the mid-ventral line. In any case the thickened border of the mantle is 

 not affected. This change in the tissues of the mantle appears to begin on each side of the middle 

 line ventrally and then to spread in all directions from the two starting points until nearly the 

 whole of the ventral part of the mantle is affected. The question. Is this an accompaniment of 

 senility ? can not help but be suggested. 



Speaking more exactly, the preceding description applies to the greater part of the ventral 

 portion of the mantle, but not to all of it. There is a narrow posterior region which is thickened 

 and which has hitherto been described as being part of the body wall. I wish to call especial 

 attention to the fact that the posterior portion of the mantle of Nautilus is a true outfolding of 

 the. body wall, and that the renal sacs and the rectum are situated within this portion of the 

 mantle and not inside the general body, as in the Dibranchiata. The pallial complex of Nautilus 

 is entirely different from that of the Dibranchiata, not alone in its parts, but also in the relation 

 of these parts to the body and to each other. This will be brought out as we proceed with the 

 description of the separate parts of the complex. 



As the renal sacs lie in the posterior part of the mantle, this is consequently thickened and 

 entirely different in its appearance from the anterior part. The renal sacs occupy only the cen- 

 tral or most ventral region of the posterior portion of the mantle. They lie entirely between 

 the posterior pair of gills. When the mantle is turned back and the animal turned ventral side 

 up, as is usually done in examining these parts (Figs. 3 and -1), the renal organs sink more or 

 less into the body, and the inner side of the mantle above them sinks down until flush with the 

 surface of the body, so that the real relation of the parts is obscured. I presume that it is owing 

 to this fact that the true position of the renal organs has been overlooked for so long by most 

 observers. I can not understand from his words whether Willey (1895) recognizes the same 

 relations between the parts of the pallial complex as I do or not. However, if one cuts the 

 mantle of a well-preserved specimen in the mid-ventral line while the animal is held in its natural 

 position, there is no doubt whatever about the extent of the mantle nor of the positions of the 

 renal sacs, rectum, anus, and gills, as well. The inner and outer walls of the posterior portion 

 of the mantle fold are very thin and soft. Keferstein described the thickened portion of the 

 mantle as a projecting part of the body wall forming a posterior wall to the mantle cavity, upon 

 which were located the anus, gills, and nephridial and pericardial pores. Owen describes the 

 gills as situated upon the mantle. Joubin, however, certainly recognizes the real extent of the 

 mantle and the position of the various parts of the pallial complex. 



