1897.] NATURAL SCIENCES OF PHILADELPHIA. 301 



are probably homologous to the small ones of the general mantle 

 surface but, unlike them, consist of but one calcareous shaft im- 

 bedded in chitinous matrix which remains after decalcification. 



Pilsbry 3 has placed the Cryptoehiton as an offshoot from the Acan- 

 thochitoid stock. In the Acanthochitidse there are always sutural 

 tufts or groups of spines corresponding in number and position to 

 the valves of the shell. In this relation it is interesting to note that 

 we have a similar arrangement in the young C. stelleri. On each 

 side of the mid-line are tufts opposite each valve that are larger 

 than any others in this region, and show a distinct regularity of 

 position (Plate VIII, fig. 3). There are multitudes of other groups, 

 but they are scattered irregularly over the mantle and do not in 

 any way correspond to the position of the valves. 



Along the median dorsal line, corresponding to the posterior por- 

 tion of each valve, is a series of eight openings. They are about 

 0.5 mm. in diameter and through them the shell is plainly visible. 

 Carefully dissecting out the valves, which are pure white and of the 

 same shape as those of older specimens, a well defined tegmentum is 

 visible (Plate VIII, fig. 2). In most cases it exhibits growth lines 

 and obscure ribbing of no definite pattern. The color is generally 

 white, sometimes slightly greenish, and is generally tinted posteriorly 

 and in some cases anteriorly with light red pigment. No tegmental 

 sense organs (aesthetes) were seen. 



On the valves of specimens almost fully grown, in a position corre- 

 sponding to the tegmentum on young shells, one may see a small 

 brown spot of about 0.5 mm. diameter which Middendorff* named 

 the nabel or umbo. This investigator figures a section where a plug 

 of some brown organic material something like chitiu projects from 

 the umbo about half way through the mantle. This is covered 

 by columnar or pavement epithelium and while it is spoken of as 

 the "navel" by Middendorff, he states that he is unable to ex- 

 plain its significance. It beyond doubt represents the opening 

 through the mantle as found in young specimens and consists of the 

 degenerate tegmentum over which the mantle has closed and par- 

 tially fused. 



Thus it is seen that no Chiton so far as known exhibits through- 

 out life a condition where the shell is wholly concealed within the 

 mantle. By its development we are justified in saying that Orypto- 



3 Manual of Conchology. Vol. XV, Part 57. 



* Mem. de PAcad. St. Petersbourg. Tome VI, 1849. 



