228 



which to describe his new species, and the figure given is a 

 ratlier faulty representation of his type. Material obtained 

 since furnishes the following information, and establishes it 

 as a true species. 



The dimensions given are: — Antero-posterior, 20 mm.; 

 umbo-ventral, 18; sectional diameter, 10'5. My largest ex- 

 ample measures 36 by 37 by 19'5. As it grows it therefore 

 tends to become comparatively shorter antero-posteriorly, and 

 more obese in section. Two individuals, each 32*5 and 33 

 mm. in the above measurements, have respectively sectional 

 diameters of 20 and 16 mm., demonstrating very great dif- 

 ference in ventricosity. 



There is a marked tendency to rest-periods in its growth, 

 producing, at intervals of about 5 millimetres, concentric steps 

 from a half to one millimetre in depth. There are usually 

 four of these in the adult ; there may be six. Then they 

 rapidly become closer and less valid, until the senile stage 

 is reached, when the shell increases much in obesity and very 

 little in altitude, and they are reduced to close-set concen- 

 tric striae. 



The species is appropriately designated "sordidus," for 

 most shells show scarcely any colour markings on their dirty- 

 white surface. But some are irregularly sparsely dotted with 

 reddish-brown, in somewhat zigzag concentric lines, and are 

 ornamented with four broad, curved, dark purple-brown 

 flames, crossing the anterior and posterior marginal areas of 

 the shell. 



Thracia myodoroides, E. A. Smith. 



Thracia myodoroides, E. A. Smith, Chall. Zool., 1885, vol. 

 xiii., p. 70, pi. vi., f. 6-6b. Type locality, off East Moncceur Is- 

 land, Bass Strait, in 38 to 40 fathoms : Pritchard & Gatliff, Proc. 

 Roy. Soc. Vict., 1903, vol. xvi (n.s.), part i., p. 104. 



This shell was recorded for South Australia by Professor 

 Tate under the name of Thracia modesfa, Angas, from Tap- 

 ley Shoal, in Trans. Roy. Soc. S. Austr., 1888, vol. xii., p. 67. 



It has been dredged alive at all depths between 8 and 

 22 fathoms ; most abundant a.t 20 to 22 fathoms. Valves 

 are found at all depths up to 60 fathoms ; most numerous be- 

 tween 15 and 22 fathoms. It is fairly abundant, and is dis- 

 tributed from Beachport westward through Spencer and St. 

 Vincent Gulfs. 



As the shell grows larger, it changes in shape, becoming 

 proportionally longer antero-posteriorly; thus the type is 13 

 mm. long by 9*5 mm. high. Other individuals are 16 by 

 10 and 18 by 11, whereas they should be 11*6 and 13*1 in 

 height respectively. 



