30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



heim was validly proposed in 1807 before Lopha was validated with 

 formal description. The new rules of the Commission, however, 

 published in 1961, do not demand formal description of a new genus 

 as long as it is accompanied by listing of a valid species. Thus Lopha 

 can be considered validly proposed as of 1798, and Alectryonia, 

 proposed later with the same type species, becomes a junior objective 

 synonym. 



The restricted usage of Lopha here does not incorporate many 

 groups that have previously been assigned to the genus because of 

 their plicate shell, but which probably belong to distinct groups. 

 Among these are the plicate Pycnodonte, which have vesicular shells, 

 Arctostrea, a distinct lineage since the Jurassic, Agerostrea, a modifi- 

 cation of the Arctostrea branch, and the narrow elongate "tree 

 oysters" with their clasping shelly processes. 



In published descriptions of Lopha or Alectryonia, certain mor- 

 phologic features are given as diagnostic of the group which I do not 

 consider significant at this taxonomic level. Curvature of the adult 

 hinge line is too easily distorted by variation in direction of adult 

 growth to be useful, especially for oysters growing in crowded con- 

 ditions. The divaricate pattern of radiating ornamentation attributed 

 to Alectryonia and Lopha is not characteristic of the genus but rather 

 of Arctostrea and some Rastellum. The use of this as a diagnostic 

 generic character reflects the time when these groups were all placed 

 under Lopha. Clasping shelly processes are rarely developed around 

 the attachment area of Lopha and are not diagnostic. They are 

 more typical of the so-called tree oysters, which probably constitute 

 a distinct group. 



Characters of the shell which appear to be useful in distinguishing 

 Lopha from similar forms, and in separating the genera of plicate 

 oysters, are: General shell form; basic ornament pattern; relative 

 development of component parts of the cardinal area; presence, 

 morphology, and distribution of the denticles ; presence or absence of 

 subcardinal cavities; depth of the valves (especially the left valve) ; 

 position, size, and shape of the posterior adductor muscle scar; shell 

 structure ; and in some cases juvenile ornamentation and nature of the 

 commissure. 



LOPHA LUGUBRIS Conrad 

 Plate 1, figures 1-18; plate 2, figures 1-17; plate 8, figure 12 



Ostrea lugubris Conrad, U. S. Mex. Boundary Rep., vol. 1, p. 156, pi. 10, 

 figs. 5a, b, 1857.— CoQUAND, Mon. Genre Ostrea Terr. Cret., p. 66, pi. 36, 



