Suter. — On New Zealand MoIIusca. 237 



Diplodon menziesi, Gray, subsp. hochstetteri, Dunker 



(1862). 



TJnio hochstetteri, Dunker, Malak. Blatter, vol. viii., 1862, p. 153. 



The type was collected by Hochstetter in the Waikato River. 



(1.) Lake Tawpo (Stat. 46 — From dredgings in 10-30 ft.). — 

 Six specimens, blackish-brown, much corroded round the beaks, 

 most of them more produced and rounded posteriorly than the 

 type, but some have the posterior end distinctly truncated and 

 more or less biangular. In correspondence with Mr. C. T. Simp- 

 son I expressed the opinion that hochstetteri is a pathological 

 subspecies,* having seen the same form amongst specimens of 

 rugatus, Hutton, from the Kopuaranga River, f and also amongst 

 menziesi from the River Avon. The young shells are invariably 

 typical D. menziesi, but on growing larger the deformity con- 

 stituting hochstetteri becomes more and more apparent. Some 

 specimens are more affected, others less, thus producing a strongly 

 truncated biangular posterior margin, or it remains only flatly 

 rounded. At the posterior end the periostracum is produced in 

 thick, foliated layers, and the inner margin is considerably 

 thickened by pearly substance, forming large rugosities, and 

 very often pearls adhering to the shell are met with. Loose 

 small pearls of irregular form are only exceptionally found. In 

 my opinion the cause of this is most likely some parasitic crea- 

 ture, as is the case in most of the pearl-producing bivalves. The 

 outer exposed layers round the beaks are smooth, light-brown, 

 waxy. The concentric striation is rather coarse, the marks of 

 rest distinct and elevated. The inside is but little iridescent, 

 except along the ventral margin, outside the mantle impression, 

 grey to light-brown, sometimes blotched with brown ; there are 

 more or less considerable rugosities beyond those at the posterior 

 margin. The dorsal scars in the umbonal cavity are small and 

 deep, and to the number of two to four. The a2iterior adductor 

 impressions are irregular in shape, and much deeper than the pos- 

 terior ones, which are oval and shallow. Right valve with two 

 pseudocardinals, the upper anterior forming a small lamella, 

 the posterior being strong, compressed, high, triangular, and 

 slightly corrugated. The single lateral tooth is regularly slightly 

 curved and somewhat crenate at its posterior end, which is 

 abruptly descending. Left valve with two pseudocardinals 

 arising from a common base, both blunt, the anterior tooth 

 larger and but slightly crenate. The two laterals are also curved, 

 the lower of them is strongly lamellated and denticulated at its 

 posterior end, which slopes down very gradually. 



* C. T. Simpson, " Synopsis of the Naiades," p. 889 (footnote), 

 f Trans. N.Z. Inst., xxiv., 275. 



