358 ROBERT TRACY JACKSON ON THE 



to eighty-four heart-beats per minute in sevei'al counts. At other times in the same 

 specimen forty-four, forty-two and twenty-one beats were counted; but these were much 

 below the normal. In a younger specimen sixty beats were I'ecorded at several counts. 

 Watching the heart it may be seen to stop for a considerable time and then begin again 

 first beating slowly; but soon getting up to its normal lapidity. The cessation of pulsa- 

 tions may be induced by disturbing the animal and the pulsations are most regular and 

 active while the Anomia is quietly feeding with the mantle extended. 



The gills in very young Anomias the age of PI. xxix,fig. 5,are simple, straight filaments 

 not joined to one another in any way and the tips not yet recurved upon themselves. 

 (Compare with young oyster gills, PI. xxiv, fig. 6.) Dissecting specimens of the age of 

 PI. XXIX, fig. l,itAvas found that the inner pair of gill plates had their filaments reflected 

 dorsally and the filaments of the opposed lamellae were joined to one another by the con- 

 crescence of their dorsal borders. The two outer gills, on the other hand, had their fila- 

 ments reflected upon themselves; but the recurved i)art equalled only about half the 

 length of the direct part. The same lelation of filaments exists in the adult gill diagram- 

 matically shown in PI. xxiv, fig. 11. The individual gill filaments in specimens of the age 

 of fig. 1, as also in the adult, are joined to one another by concrescence at the tips of the 

 dorsally reflected borders; but they are not joined to one another at other points. Further 

 the filaments have not the nodular ciliated processes characteristic of the gills of Pecten . 

 Therefore, the gills of Anomia hav^e filaments which are individually free to a somewhat 

 remarkable extent. This is interesting as showing an almost primitive condition in one 

 organ when other organs are so modified as to render Anomia one of the most highly 

 specialized of the Pelecypoda. The gills are connected with the body by two membranes, 

 s, PI. XXIX, figs. 1-2, which have been desci-ibed (Tryon) as falcifoi-m. These membranes 

 are attached to the adductor muscle, to the body anteriorly from that point and to the 

 mantle wall posterioi'ly to the adductor. The posterior attachment is not shown in La- 

 caze-Duthiers' figures; but it is a marked feature in the young of Anomia glabra. A 

 large foramen exists in each of these membranes. The foramen of the left membrane, fig. 

 1, is the larger and my attention was attracted to it by seeing the extensible and restless 

 foot prolonged across the body and thrust through this hole. In fact it was a favorite act 

 with this individual at least, for after noticing it I frequently found the foot thrust through 

 the foramen. The foi-amen in the right gill membrane, fig. 2, is smaller than that in the left. 

 The membranes extend in a free curving line to the posterior extremity of the gills. The 

 gills are capable of considerable motion and frequently in the young may be seen pro- 

 truded beyond the margins of the valves and even beyond the borders of the extended 

 mantle lobes as well. The length and width of the several gills is different as pointed out 

 by Lacaze-Duthiers. In comparing PI. xxix, figs. 1-2, it is seen that the gills on the left 

 side are much longer and broader than on the right side on which the animal rests, a fea- 

 ture which, as Lacaze-Duthiers said, is doubtless connected with the conditions of fixation. 



The mantle of Anomia presents pecidiarities which are necessitated by tiie al)noi'mal 

 position of the byssus. The mantle lobes are free except for a short distance on the liga- 

 mental area (observed in an adult) where the lobes of the two sides are confluent. The 

 right lobe has a deep re-entrant fold where it surrounds the b3'ssus, shown in Lacaze- 

 Duthiers' figures and slightly indicated in PI. xxix, fig. 2. The margins are frijiged 



