53 R. HOLMAN PECK. 



comprising in its wall a definite number of filaments. A 

 series of diagrams illustrating the general arrangement of 

 the filaments in these cases of which there are many varieties 

 (e.g.fOstYea, Cardium,Venus, Solen, Pinna) has been given by 

 Posner. I shall defer any further consideration of these 

 modifications of the Lamollibranch gill until I have com- 

 pleted my examination of those forms at my disposal, merely 

 pointing out now that the inner gill-plates of Anodon present 

 a distinct tendency towards the fluted type or " compound 

 type" of gill as it has been called, which the outer gill-plates 

 do not. This interesting fact has escaped Posner. 



Details of the Structure of the Gill-plates in 

 SPECIAL Genera with reference to the Figures in 

 Plates IV, V, VI. 



Arca. — The gill-plates in Area exhibit more clearly the 

 primitive freedom of the typical two rows of reflected fila- 

 ments than do those of any other genus, the examination of 

 which has been recorded. There is but small manifestation 

 here of the phenomenon of '' concrescence." The inner re- 

 flected filaments are free from adhesion to the foot, the outer are 

 free from adhesion to the mantle. Not only this but there is 

 no concrescence of the two inner reflected lamellae or rows of 

 filaments behind the foot. The longitudinal lophophoral 

 ridges on either side the foot from which the descending 

 limbs of the filaments spring are continued beyond the foot 

 as a pair of short spurs with a slight curvature, and are 

 directly comparable to the lateral " arms" of the Brachiopod's 

 or the Ilippocrepian lophosphore. The only concrescence 

 which I have detected in an examination of specimens from 

 the Mediterranean, preserved in strong spirit, is at the apex 

 or angle of the lamellae where the reflected portion is united 

 for a short distance by concrescence to the descending portion 

 of each filament. 



The filaments in large specimens of Arca (one and a half 

 inches long) where longest measure as much as three quarters 

 of an inch from base to apex; towards the anterior termina- 

 tion of the series they gradually become very short as well 

 as at the free posterior projection of the lophophore. Each 

 filament is rather band -like than cylindrical, the broad sur- 

 faces of the bands being anterior and posterior, whilst the 

 narrow edges are lateral and medial or external and internal. 

 On the broad surfaces of the filaments are placed at regular 

 intervals the "ciliated inter-filamentar junctions" so as to form 

 slightly undulating oblique transverse rows when the lamella 

 is viewed as a whole. The ciliated junctions consist of large 



