217 



Dentalium hyperhemileuron, n. sp. PI. xxvi., 

 figs. 3 and 3a. 



Shell long and narrow, very slightly curved, mostly at 

 the hinder part, white when dead, translucent when fresh, 

 and glistening, rather thin. There are 12 axial ribs, invalid, 

 and narrow ; no increase in number with age ; becoming 

 obsolete early, so as to leave the anterior two-thirds of shell 

 smooth but for very fine accremental scratch lines. Inter- 

 stices nearly flat, slightly concave. Anterior orifice round, 

 margin thin and simple. Posterior end truncated, with a 

 long narrow diaphanous appendix directed eccentrically dor- 

 sally. The growth lines on the appendix form a convexly 

 bordered sinus on the ventral surface about as wide as deep, 

 and a scarcely depressed margin on the dorsal surface. 



Dimensions. — Length, 20'5 mm. ; greatest width, 1*8 

 mm.; least width, '7 mm.;, length of appendix, 2'2 mm.; 

 diameter, *4 mm. 



Locality. — King George Sound, Western Australia, in 

 12-14 fathoms, 200, several alive; in 22-28 fathoms, 60, 

 several alive; in 35 fathoms, 4 dead but good; Geographe 

 Bay in 15 fathoms, 6 dead but good; in 22 fathoms, 4 dead; 

 off Fremantle in 10-12 fathoms, 20 poor. 



Some individuals with perfect posterior ends run down 

 to a diameter of '3 mm., and are there diaphanous and ribless, 

 and have only growth striae. Others more mature and with 

 a posterior end of 1 mm. in diameter, and without an 

 appendix, are here bevelled internally and thinner on the 

 convex side, where there is a shallow triangular notch. The 

 largest example is 30'75 mm. long by 2'3 mm. wide. The 

 ribs may vary in number from 10 to 16 in different indi- 

 viduals. 



Diagnosis. — It very closely resembles D. hemileuron. 

 Verco, in the ribless anterior portion and the never-increasing 

 ribs of the posterior end, and in their extension to within 

 2 mm. of the end in very young individuals and in the 

 ventral notch at the hinder extremity ; but the latter has no 

 appendix, and the ribs are more valid and do not so soon 

 become obsolete, and it is not found in such shallow water. 

 But I think probably the absence of the appendix may be 

 only an accidental circumstance, and the shallower water in 

 which the Western Australian species lives may account for 

 the other differences, and that this is only a local variety. 



One individual, dredged in Geographe Bay at a depth 

 of 15 fathoms, measures 4 mm. in length by '5 mm. in 

 diameter at the- anterior end. It has the apical end com- 

 plete. The first portion of this, measuring 1*9 mm., has been 



