6o8 C. B. Daveuport and Marian E. Hubbard. 



the Mediterranean coast of France. Dr. Louis Calvet, in charge 

 of Marine Zoological Station of the Universite de Montpellier at 

 Cette, kindly arranged for the collecting of these shells and the 

 shipping of them to America.^ 



The counting of the rays of the shells was done by the junior 

 author after a preliminary joint count had been made on a num- 

 ber of identical shells to decide upon the standards to be adopted 

 in counting. Since the interior grooves corresponding to the 

 exterior ridges are not always clearly marked the external ridges 

 were counted. Now, the counting of rays in Aequipecten was 

 made on the interior grooves, and since there are always a few ^ 

 lateral ridges not represented by interior grooves (and so not 

 enumerated) the averages cannot be too closely contrasted with 

 those of Aequipecten, although decidedly greater. A slight diffi- 

 culty is introduced by the fact that at the lateral ends of the series, 

 rays are sometimes imperfect or ambiguous; the rule adopted was 

 this: all rays, however faint, that reach the margin of the shell 

 were counted in provided they extended centrad beyond the distal 

 limit of the "ears." The shells having been numbered, the count 

 for each individual shell was recorded and later each determina- 

 tion was checked by a recount. 



The data were seriated and subjected to the usual quantitative 

 analysis. All calculations were checked by being made indepen- 

 dently by each of us. 



III. RESULTS. 



The most striking difference between the system of rays of P. 

 varius and that of P. opercularis-irradians type is found in the 

 greater number of irregularities in the rays of the former. Such 

 irregularities are relatively rare in P. irradians (Davenport, 

 1900^, p. 879); commoner in P. opercidaris (1903^^, p. 138); but 

 are found in a large percentage of the shells of P. varius. About 

 12J per cent have rays double, or one ray interpolated, or one, as 



" I take this opportunity to acknowledge the generosity with which the privileges 

 of the beautiful Station at Cette were accorded Mrs. Davenport and myself in 

 November, 1902, by Professor Sabatier, the Director of the Station, and Dr. Calvet. — 

 C. B. Davenport. 



2 One to five. 



