KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. NIO 3- 25 



mentioned and drawn by Pelseneer (1911). In a somewhat contradictory manner 

 this aufchor designates it on page 58 as being a »jeune indéterminable», but on the 

 next page 59 as being »å maturité sexuelle»; the latter statement is also supported 

 by the figure (pl. XXI, fig. 7), in which the genital glands are reproduced as highly 

 developed (ef. below, anatomy). The length of this animal is about 5 mm. 



To judge from the liinge construetion of this new form (the presence in the 

 left valve of a weak anterior cardinal tooth, of a stronger posterior one and of a 

 marginal tooth) as well as from the mode of attachment (by means of the right 

 valve) it is to be included in the new genus Pseudochama. Besides this fact, we 

 shall find that its hinge exhibits characteristics which do not persist in the adult 

 species of the genus hitherto known, but which prevail during an earlier stage of 

 their development (separation of 1 and 3 b and persist ence of LP II). Compared 

 with other species of the genus Pseudochama pusilla, consequently, preserves juvenile 

 characteristics during its whole life, a circumstance giving us a reason for establishing 

 a distinct sub-genus for the new species. As a significant sub-generic name for it 

 I propose the combination Eopseuma. 



In three of the specimens the soft parts of the animals were well preserved 

 and enabled a detailed examination of the anatomy to be made. 



Externa] inornholoffy of the animal (figs. 22, 23). 



The mantle consists of a simple layer of flat epithelial cells. Its inferior 

 margins are coalesced all round, except for the three openings of the siphons and the 

 foot; the zone of coalescence and the free mantle edges outside it are wholly smooth, 

 but the short, somewhat separated siphons are slightly papillated; their openings 

 occupy together a length of about a third of the under side (= distance between 

 the lower ends of the adduetors). The small pedal slit is situated immediately be- 

 neath and in front of the anterior adduetor, and above it the pallial margins are 

 coalesced again. 



The visceral hump exhibits regular symmetry except that the right umbo is 

 more elevated than the left one. In this elevation the right gill and the dorsal 

 inner organs also participate, but beneath the level of the heart at the point where 

 it is penetrated by the rectum as well as beneath the upper part of the stomach 

 there is, for the most part, a symmetrical organization of the animal throughont. 



From the under frontal side of the visceral hump projects the small finger-like 

 foot, somewhat bent upwards; its length is about 2 / 3 °f that of the anterior ad- 

 duetor. In transversal section it has an elongated elliptic outline, due to its com- 

 pression from the sides. On sections only small traces of the posterior retractors of 

 the foot are perceptible; these penetrate the nephridia as thin threads, attaining 

 only half the breadth of the nerve cords. No anterior retractors exist. On the 

 posterior side of the visceral hump an indistinct triangulär cutaneous lobe projects 

 in the median line beneath the genital openings. 



K. Sv. Vet. Akad. Handl. Band 59. N:o 3. 4 



