204 DE. c. T. TEECHMAy>r o^ [vol. Ixxiii^ 



Material. — I collected some twenty-five specimens, several of 

 Avliich bear a portion of the shell. The I^ew Zealand Geological 

 Survey has many specimens, mostly in the form of casts. They 

 generally occm* in a very unsatisfactory state of preservation, with 

 most or all of the shell dissolved a\va3\ 



Locality and horizon. — The genus seems to be confined to- 

 the Carnic beds and occurs in all localities, such as the Nelson 

 district, the Hokonui Hills, Xugget Point, etc. Separated valves- 

 are found in the 3Iyfihis-prohlemaficus Bed. 



Affinities. — I found considerable difficulty in attempting to 

 determine the relationship^ of these curious bivalves. The anterior 

 tongue-like projection of the right valve appears to be homologous 

 Avith the anterior auricle of the Myalinidse, to which group the 

 genus seems to belong. The anterior retreat or thickening of the- 

 margin of the left valve below the beak resembles that of Per- 

 c/amidia Bittner,! a genus which occurs in the Upper Trias of 

 Balia in Asia Minor ; but in Holtonuia it extends over a less 

 distance of the anterior margin than it does in J^ergamidia. 



The genus Mysidia Bittner ^ occurs also in the Upper Trias of 

 Balia : but only right valves were known to Bittner, and they have 

 a prominent tooth beneath the beak. 



Hokonuia also recalls the genus jSIijddiojyiera Salomon, a group- 

 of bivalves described as Mytiloid Limidse, which occur in the 

 Upper Trias of the Alps and other regions. In these, however, 

 according to Bittner's illustrations, the bj'ssal opening is similar in 

 both \'alves. As a result of later research, however, Bittner shows- 

 that in Mysidioptera the area of the right valve is highei' than 

 that of the left. 



The anterior ear of the right valve in ILol:onuia recalls in some 

 ways that which characterizes the left valve of Eiirydesma, a form 

 occurring in the Pernio- Carboniferous marine beds of Australia 

 and other regions of Gondwanaland. 



As I can find no genus closely comparable with these New 

 Zealand shells, I am compelled to institute a new generic name for 

 them, and to regard them as having affinities with l^ergawidia^ 

 JMysidia, and Mysidioptera, three Z/;;iff-like Myalinidae of the 

 Upper Trias of the Alps and Asia Minor. 



It seems that a considei'able number of species occur in New 

 Zealand: but, owing to the poor state of preservation of the material,, 

 it is very difficult to separate these, or accurately to diagnose them. 

 I offer descriptions of two species which seem to be the commonest 

 and most distinctive. 



HoKOXuiA LiM^FOEMis, sp. nov. (PI. XXII. figs. 2 <7, 2 5, & 5 ;: 

 PL XX, fig. 4.) 



Shell rather higher than long, hinge-line straight, measuring 

 about two-thirds of the length of the shell. Beak lather pointed,. 



1 Bibliography, 6. xli, p. 103 & pi. iii, figs. 1-4. 

 - Ihid. p. 113 & pi. ii, fig. 10. 



