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Volume 37, 1904
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Diplodon lessoni, Küster (1856).

Unio lessoni, Küster, Conch. Cat., 1856, p. 135, pl. xxxvi., fig. 4. Type from New South Wales.

Lake Wakatipu (Stat. 37—Dredged in 10 ft. to 30 ft.).—Nine specimens were obtained. Shell (fig. 4) oblong, obliquely truncated behind, compressed, inequilateral, beaks strongly eroded in all specimens, no trace of sculpture left; surface with distant flatly elevated rest-marks, which, together with the intervening space, are covered with very fine thread-like concentric lines; towards the base and posterior margin the growth-lines are more distinct and slightly foliated. There is no trace of radial sculpture. The epidermis is olive to dark-brown, dull. Interior bluish nacre, with yellowish patches under the beaks, where there are several deep small dorsal scars. Right valve: The pseudocardinals are compressed, the anterior upper tooth is small, grooved, the posterior tooth much larger and crenate; the lateral tooth is slightly arched and rugose posteriorly. Left valve: Anterior pseudo-cardinal compressed, tongue-shaped, slightly rugose, the posterior subtriangular, deeply denticulate; the laterals are of nearly equal height and crenate posteriorly.

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Fig. 4

The largest specimen was selected for the diagram and description, and its dimensions are—Length, 51 mm.; height, 30 mm.; diam., 14½ mm.

I submitted specimens to Dr. W. H. Dall, Hon. Curator of Mollusks, U.S. Nat. Museum, for examination, and he very kindly informed me that according to the material in the museum they are D. lessoni, Küster. I have not seen any Australian specimens, nor the description and figure published by Küster, but I do not hesitate to accept Dr. Dall's view, although some may think it hazardous to refer our shell to a New South Wales species. I have not seen this species from anywhere else in New Zealand.

Dr. Von Jhering once suggested that a number of measurements should be taken to ascertain the range of variability in species of the family Unionidœ. I have measured a great number of specimens from New Zealand some years ago, and may publish the results later on. I have done the same for a number of the Unionidœ collected by Messrs. Lucas and Hodgkin, and

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the result is contained in the following table. [The ciphers i. to xi. in the table indicate—i. Length of shell. ii. Greatest height. iii. Index of height (length = 100, height in % of it). iv. Distance of greatest height from anterior end. v. Index of position of greatest height (length = 100, distance of greatest height in % of it). vi. Diameter. vii. Index of diameter (length = 100, diameter in % of it). viii. Length of hinge line. ix. Distance of beak from anterior end of hinge line. x. Umbonal index (length of hinge line = 100, distance of beak from anterior end of hinge line in % of it). xi. Cardinal index (length of shell = 100, length of hinge line in % of it).]

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

— i. ii. iii. iv. v. vi. vii. viii. ix. x. xi.
D. menziesi.
Taupo, 9 32 19 59 9 28
" 9F 37 23 62 10 27
Waikaremoana, 14 44 28 64 23 52 13 29 32 9 28 72
" 14 41 23 56 25 61 10 24 25 6 24 61
" 14 39 22 56 21 53 10 27 24 6 25 61
Mean of the three above 41 24 58 23 55 11 27 27 7 25 64
Waikaremoana, 37 55 32 58 30 54 18 32 38 10 26 69
" 37 60 33 55 28 47 17 28 40 10 25 67
" 37 61 36 59 31 50 19 31 40 10 25 66
" 37 62 33 53 31 50 19 30 40 10 25 64
Mean of the four above 59 33 56 30 50 18 30 39 10 25 64
Cotype of waikarense 79 48 61 40 51 25 31 47 10 21 59
D. hochstetteri.
Taupo, 46 48 31 65 24 50 16 33 36 8½ 24 75
" 46 52 36 70 30 58 18 35 35 8 23 67
" 46 54 36 67 30 56 19 35 40 10 25 74
" 46 64 40 63 33 52 22 34 48 11 23 75
Mean of the four above 55 38 66 29 54 19 34 40 9½ 24 72
From Lake Rotorua, typical 61 44 72 32 52 24 39 42 10 24 69
D. rugata.
Waikare, 41 47 28 59 22 47 14 29 31 8 26 66
" 41 46 29 63 26 56 15 32 30 8 26 65
" 41 44 26 59 20 45 13 29 25 6½ 26 57
" 41 41 25 61 21 51 13 31 29 8 27 70
Mean of the four above 44 27 60 22 50 14 30 29 8 26 64
From Lake Pearson, cotype 52 33 63 26 50 17 32 35 9 25 67
D. lucasi.
Manapouri, 35 45 24 53 19 42 8 18 28 8 28 62
" 35 34 21 61 16 47 8 23 22 7 31 64
" 35 28 17½ 62 15 53 6½ 23 18 5 28 64
D. lessoni.
Wakatipu, 37 51 30 58 23 45 14½ 28 35 9 26 68
" 37 44 24 58 22 53 11½ 28 26 7½ 28 63
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