260 



LOWER INVERTEBRATES. 



are found attached to the roots of mangrove and other trees, in the warmer seas of 

 the world. The oysters appeared in the carboniferous age, and have persisted until 

 the present time. Among the fossil forms closely allied to Ostrea 

 may be mentioned the genera Gryphma and Exogyra, in which 

 the shell attained great thickness and 

 weight. 



The genus Anomia, though pos- 

 sessing but little economic importance, 

 „„„ ,. is especially interesting from the fact 



Fig. 268. — A ery young f j to 



Anomia, showing that for a long time it was supposed 



al notoli ° ^'^ 



the byssal 



not yet conveited to form a connecting; link between the 



into a foramen. ^ , 



brachiopods and the molluscs. Like 

 many of those forms it lived attached to rocks or shells 

 by means of a peduncle, which perforated one of the'Fie. 269.— Anomia fpftippium, showing 

 valves of the shell. Could the correspondence be 



stronger? When the mode of growth was studied it was found that the two were en- 

 tirely different. Anomia, in its earl}' stages, spins a byssus, by which it attaches itself 

 to some foreign object. As the shell increases in size, the byssus interferes with the 

 growth of one of the valves, producing a notch in its margin. As growth continues, 

 the two edges of the notch close around the byssus, resulting in the perforation. 

 As this closure occurs at an early stage of development, the foramen produced is 

 close to the hinge, thus strengthening the resemblance to the similar opening in the 

 brachiopods, though, as will be readily seen, there is no homology between them. 



Besides the perforation of one xaXxe for the byssus, 

 the genus Anomia is characterized by having the two 

 valves unequal, one being larger and convex, while the 

 other (the perforated one) is flattened, or even concave, 

 according to the location on which it settled. The 

 hinge is without teeth. The species vary greatly, so 

 that it is very difficult to define them. On our coasts 

 two forms are commonly found, A. glabra and A. acu- 

 leata, the former being essentially a southern, the latter 

 a northern form. Anomia glabra is especially com- 

 mon on oysters. In color it is glistening white or 

 yellow, and hence has received the names siher shell 

 or gold shell. A. aculeata is usually spiny or scaly. 

 It is more varied in its places of attachment, rocks, shells, and large algSB forming 

 favorite localities. This is very closely allied to the Anomia ephippium of Europe. 

 On the coasts of California occurs A. pernoides. 



Placuna is a genus with thin shells, in which no byssus is formed. The valves are 

 nearly equal, and the species inhabit sandy shores. The species are inhabitants of the 

 Indo-Pacifio Ocean. 



The family PECTiNiDiE, or scallops, comes next in order. Here the valves may be 

 similar or different, wliile the anterior and posterior halves of each valve are nearly 

 alike. The hinge is prolonged on each side of the umbo into an ear-like process. 

 The borders of the mantle are free, and frequently they bear a number of brightly 

 colored eyes, which have been made a subject of study by Mr. Holman Peck. A 

 single adductor is present, and the gill filaments are free, showing as a permanent 



Fig. 270. — Anomia {ilabra. 



