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Ennucula georgiana Dell 1964

Description

provided by NMNH Antarctic Invertebrates

Ennucula georgiana sp.n. (pl. II, 1, 2)

OCCURRENCE: St. 27, South Georgia, 110 m; St. 30, South Georgia, 251 m, paratypes; St. 45, South Georgia, 238-270 m, paratypes; St. 123, South Georgia, 230-250 m; St. 140, South Georgia, 122-136 m, holotype; St. 142, South Georgia, 88-273 m; St. 144, South Georgia, 155-178 m, paratypes; St. 149, South Georgia, 200-234 m, paratype; St. 152, South Georgia, 245 m, paratype; MS 68, East Cumberland Bay, South Georgia, 220-247 m.

Shell of medium size for the genus, oval, beaks about posterior fifth, not greatly inflated. Dorsal and ventral margins broadly rounded. Anterior margin narrowly rounded with no angulation. Posterior margin, shorter, subangled about the middle. Sculpture consisting of fine, rather regular growth wrinkles, shell surface covered with a polished, yellowish-brown epidermis. Lunule clearly marked off by a very fine raised ridge which may be slightly lighter in colour. Hinge plate narrow, evenly curved across the upper borders, broken under the beaks. Anterior teeth 12 in number, teeth in middle of series very long, diminishing regularly towards beaks. Hinge-line interrupted by a somewhat oblique, very stout chondrophore. Posterior teeth, seven in number, the median series very stout, posterior hinge-plate subangled. Interior of the valve smooth, polished, with microscopic radials. Valve margins smooth, covered by a narrow reflection of epidermis.

Holotype, B.M. Reg. no. 1962872/1, St. 140, and paratypes, B.M. Reg. nos. 1962873/3, St. 30; 1962874/7, St. 45; 1962875/2, St. 144; 1962876/1, St. 149; 1962877/1, St. 152, in Britiish Museum (Natural History).

This form differs from those recorded from southern South America in the evenly rounded posterior margin with very ill-defined subangulation, and in the form of the chondrophore, which is rather less oblique than in most other species of Ennucula. It appears to be confined to South Georgian waters. Martens described a Nucula miniuscula from South Georgia. David (1934, p. 129) recorded that a comparison of the type of this species with specimens of Rochefortia charcoti Lamy showed that the two species are the same. Soot-Ryen (1951, p. 5) noted this fact and also recalled that Melvill & Standen (1907, p. 113) had recorded Nucula miniuscula Martens from the South Orkneys. These specimens were determined by Pfeffer (1886) and certainly belonged to Nucula, but Pfeffer's original figure of miniuscula is quite unlike georgiana and also quite unlike Melvill & Standen's specimens of 'miniuscula'.

The only other species of nuculid described from this sector is N. falklandica Preston described from the Falkland Islands. N. pisum Sowerby has also been recorded from the Falklands. These records are discussed elsewhere.

Measurements and indices for species of Ennucula

Length (mm)

Ht. (mm)

Pos. of beaks (mm)

H.I.

B.I.

Ennuncula grayi (Orbigny)

Type-series

17.9

14.0

3.7

78

21

Type-series

14.1

10.9

3.0

77

21

British Museum specimens

15.4

11.5

3.7

75

24

British Museum specimens

15.0

11.2

3.8

75

25

British Museum specimens

12.3

9.1

2.7

74

22

WS 777

19.6

15.0

4.1

77

21

Ennucula puelcha (Orbigny)

uruguayensis (type)

12.2

9.7

1.3

80

11

uruguayensis (type-series)

10.0

7.2

1.3

72

13

uruguayensis (Montevideo, Challenger)

12.8

9.4

1.7

73

13

uruguayensis (Montevideo, Challenger)

12.7

9.6

1.2

76

9

uruguayensis (Montevideo, Challenger)

11.6

8.8

1.2

76

10

uruguayensis (Rio de Janeiro, Terra Nova)

Ennucula georgiana sp.n.

Holotype

13.2

10.1

2.7

77

20

St. 14

12.6

10.0

2.5

79

19

St. 30

15.0

12.2

3.0

81

20

St. 30

13.7

10.8

2.7

79

19

St. 152

9.4

7.0

2.0

74

21

Means and extremes of height and beak indices"

H.I.

B.I.

Ennucula grayi (Orbigny)

74-78 (76)

21-25 (22)

E. puelcha (Orbigny)

72-80 (75)

9-14 (12)

E. georgiana sp.n.

74-81 (78)

19-21 (20)

(Dell, 1964: 142-143)