DENTALIUM-FISSIDENTALIUM. 83 



Sehizodentalium plurifissuratum SOWB., Proc. Malac. Soc. Lond., i, 

 p. 158, pi. 12, f. 24 (Oct. 1894). SIMROTH in Bronn's Klassen u. 

 Ordnungen des Thier-Keichs, iii, p. 375, f. 45A (1894). 



This species is type of the genus Sehizodentalium Sowerby, char- 

 acterized by having the shell Dentalium-shaped, the convex side 

 with a longitudinal series of slits in place of the usual fissure, or, in 

 other words, the fissure is interrupted by several bridges of shell. 



The conjectured mode of formation of the slits given by Sowerby 

 seems to us inadmissable. They are, in all cases, so far as present 

 knowledge goes, absorbed out of the solid shell-wall, and not left 

 open in the progress of growth as in Emarginula or Haliotis. 



In the present lack of knowledge regarding the physiologic role 

 of the slit, the uncertainty as to whether any other character is cor- 

 related with it, and the variability of its occurrence in some forms, 

 it seems to us hardly desirable to base a generic distinction upon 

 this one modification. Compare D. exuberans Locard, and Watson's 

 remarks quoted under D. capillosum, and alluded to below. 



Mr. Sowerby further writes of this interesting form as follows : 



" The shell is very like an ordinary multicostate Dentalium, but 

 distinguished by the following remarkable character. In a line 

 with the usual apical notch there are several slits on the convex 

 side, extending from the summit to about a quarter of the length of 

 the shell. I have three specimens before me, and the dimensions 

 given above are taken from the largest ; it has five perforations, the 

 first being a narrow slit about 2 mill, in length, the second and 

 third rather shorter, and the last two only about *7o mill. The 

 second specimen is about 47 mill, long, and has five slits which are 

 narrower than in the first, four of them being of nearly equal length. 

 The third specimen is a young one 28 mill, long, having only two 

 long narrow slits. A fourth specimen has been, for many years, in 

 the British Museum unnoticed ; it is nearly as large as my largest, 

 and has four slits. 



" It is, at present, uncertain how these perforations are formed. 

 It may be conjectured that when young there has been an open slit 

 or notch in the anterior margin, as in Emarginula, which has been 

 enclosed in the next stage, as in Jtimula, a succession being formed 

 and enclosed in subsequent stages. 



" The animal is very like that of Dentalium entalis, and the Rev. 

 Prof. Gwatkin has examined the radula, finding it the same as in 

 the typical Deutalium. 



