160 



six slits ; teeth strong but irregular. Interior sea-green with brown 

 in the middle. Sinus with 10 very irregular strong teeth. 



Very interesting is the covering of the girdle. The strong 

 ealcareous scales are very irregular in size and form. At the 

 sides of the valves they are small, but iu the direction of the 

 outer margin of the girdle they become larger and larger (Fig. 

 21). Their form is irregular; they have a broad base (Fig. 18, b). 

 So the mantle has a very irregular aspect. 



Now it is a very interesting fact, that this covering of scales 

 does not extend over the whole girdle. The right anterior part of 

 the girdle (Fig. 23a) shows other scales; these are much smaller 

 than the scales of the other part of the girdle, but of the same 

 unequal size and form. Fig. 24 shows a part of this region; 

 the little imbricating scales are seen frora above and from the 

 side. A comparison between the figures 24, 25, 26 and 27 shows 

 the difïerences in size very clearly. In Fig. 25 three normal 

 scales of the girdle are seen ; in Fig. 26 and 27 scales of the 

 region a of Fig. 23. All these scales are similarly magni- 

 fied. It is clear, that the normal scales are by far the 

 largest; the scales of region a of Fig. 23 however are not of 

 equal size, but larger (Fig. 26) and smaller ones (Fig. 27) occur. 

 A great number of these larger and smaller scales of region a 

 are grouped together; these groups are very distinct. The trans- 

 ition between region a and the other part of the girdle is 

 abrupt; while the normal girdle-scales have a grey colour, those 

 of region a are grey or brownish black. The difference gives 

 the impression, that in region a we have to do with an abuormal 

 formation of scales; certainty about this cannot be obtained. 



On the uuderside of the girdle we find numerous little ealca- 

 reous bars, which are straight or somewhat curved and lie close 

 together (Fig. 22). 



The gill-row extends as far as the opening of the mouth and 

 is holobranch and adanal »rait Zwischenraum". On both sides 60 

 lamellae occur. 



The genital aperture is found between the 19^^ and 20^^ gill 



