138 



furnished with a broad beak or mucro, the tegmentum 

 being continued behind this. The dorsal area is distinct and 

 broad with almost parallel sides. The portion immediately 

 in front of the mucro is usually coarsely longitudinally ribbed, 

 more or less broken. In some of the valves these ribs con- 

 tinue to the anterior margin, in others they are replaced by 

 subcutaneous lining. In this anterior portion, irregular 

 raised pustules also occur to a limited extent in some of the 

 valves. Although these irregularities exist, the chief char- 

 acter of this part of the shell is strong, longitudinal ribbing. 

 A strongly-raised, diagonal rib commences at the mucro and 

 dies away about half-way across the tegmentum dividing the 

 lateral from the pleural area. The pleural area is sculptured 

 with irregularly-shaped, rough-looking pustules. There is a 

 tendency for these to become confluent along lines parallel 

 with the ribbing in the dorsal area. In the lateral areas the 

 raised portions or granules are even more irregular in shape 

 than is the case in the pleural area. , Behind the mucro 

 and diagonal rib the subcutaneous line-marking is radial, and 

 very marked in some valves. As before stated, the mucro is 

 anterior to the posterior margin, and the posterior lobes of 

 the insertion plates unite behind the tegmentum. 



Posterior Valve. — The anterior portion of the tegmentum 

 is less pointed than is the case in the median valves, the 

 posterior part semicircular. Mucro anterior, a number of 

 dark subcutaneous streaks radiate from the mucro posteriorly 

 in a fan, with a highly-polished, semitransparent surface, but 

 it is not truly smooth ; this character occupies about one- 

 quarter of the length of the tegmentum and is fan-shaped. 

 That part of the valve behind the mucro is sculptured fairly 

 evenly with circular pustules placed more or less concentric- 

 ally. The anterior portion is longitudinally ribbed, but, in 

 addition, there are coarse irregular pustules. This valve is 

 large, measuring 4x7 mm., the tegmentum placed centrally. 



Girdle. — In the dry specimen is wrinkled, bearing neither 

 scales, hairs, or spicules; has a gelatinous or horny look, is 

 dark in colour, and, with shrinking, has curved inside the 

 shell to a width of *75 mm. The girdle is thickened over the 

 marginal portion of the insertion plates, forming a dark band 

 round the shell 1 mm. in width or with the incurved portion 

 referred to a total girdle width, in dry specimens, of 1*75 mm. 



Measurements. — Length, 18 x 8J mm. The tegmentum, 

 or exposed part, reaches a maximum width of 1J mm. by 

 1 mm. longitudinally, in one of the median valves, whereas 

 the articulamentum in valve 6 is 3 mm., longitudinally, by 

 8 mm. in width. 



Habitat. — The specimen under review was found near 

 Cape Banks Lighthouse, in South Australia. The following 



