364 Mr. Robert Garner's Malacological Notes. 



between the anterior and posterior portions of the valves in 

 Lamellibranchiata, are facts which seem to favour the idea 

 that the dorsal valve, for example, of a brachiopod is not 

 analogous to the right or left valve of an ordinary bivalve, 

 but rather to the anterior portions of the two valves united 

 together, and so vice versd. 



The fossil Hippurites and Rudista, with such forms as 

 Dianchora and Podo])sis^ appear to have had most affinity 

 in form and structure, amongst bivalves, with the Cham ada3 — 

 branches of the same stock, one passed away, the other still 

 flourishing, therefore the affinity rather collateral than deriva- 

 tive. One or both valves may have put on the spiral form, 

 the former case in the Spherulite, and the latter in Diceras ; or 

 one of them may have become elongated and multilocular, as in 

 the Hippurite and, indeed, in some oysters. Such a spiral form, 

 or such an elongation, must be attended with a division of the 

 connecting ligament or cartilage, just as we see in Isocardia 

 or, in a different form, in Gryj^hcea. Further, if the shell- 

 forming mantle becomes expanded at the circumference all 

 round, as in Chama^ there will be a tendency for both hinge- 

 teeth and cartilage to become central, a circumstance which 

 has apparently taken place in many of these curious fossils. 

 They have other peculiarities of the valves difficult to account 

 for. 



But however explained, whether from descent, relationship, 

 or modification for or by externals, the Anomia appears to be 

 a transitional form between the attached brachiopod {Crania^ 

 Orhicula) and the ordinary bivalve ; the oblique position of the 

 mouth, the non-marginal situation of the shell-nucleus, the 

 very short intestine, the lengthened, narrow, and loose palps 

 or labial appendages (which are confluent with the branchise), 

 the complex muscular system, the ovigerous mantle, and 

 the notched valve would seem to show as much, as well as 

 the structure of the shell in some allied species. The plug 

 may be the homologue of the byssus or pedal plate of Area on 

 the one side, and of the brachiopodous pedicle and deltidium 

 on the other. A very small foot exists ; but there is a very 

 long crystalline body, having apparently a mechanical use to 

 support the unusually detached mantle-lobe. We see in these 

 transitional bivalves a tendency to diverge from the symmetry 

 which preponderates in the class throughout ; for when certain 

 other species show the same tendency, it is generally with 

 some relation to the nature of the hinge. Though the brachio- 

 pod is very distinct in many ways from the bivalve, we are 

 not convinced that the arms of the former do not perform the 

 office of branchias. 



