A. E. Verrill — Study of the family Pectinidce. 45 



Remarks on the Ontogeny and Phyloc/eny of the existing genera of 



Pectin idee. 



It is not my intention to discuss the special phylogeny of the fam- 

 ily, as a whole, at this time, for I have not at hand a sufficient num- 

 ber of the palaeozoic genera, in good preservation. This subject has 

 been discussed, to some extent, by Dr. Jackson' and others. It is 

 certain that as we go back to the Palaeozoic forms, the Pectinidse, 

 Pernidae, and other related families gi'adually converge, and give 

 evidence of a common ancestry. Dr. Jackson has suggested that a 

 Nucida-WVo. genus must have been the common ancestral form from 

 which many families of Bivalvia, including Pectinidae, Aviculidae, 

 etc., were derived. This view, which has been adopted by Prof. 

 Hyatt,'^ seems to me to lack demonstration and to be improbable, for 

 Nucula, although an ancient genus, dating from the Palaeozoic, is a 

 rather highly specialized form, as to its foot, palpi, and some other 

 parts, while its hinge-teeth are far more specialized than those of Cu- 

 euUcea, and many other early Palaeozoic forms. The veliger-stages 

 of Mytilus, Nucida, or of Perna present more nearly the forms 

 and characters which I believe pertained to the earliest bivalves 

 than any that we yet know from adult forms. But the early 

 veliger-shell of Pectinidae is similar, and perhaps nearl}^ as primi- 

 tive in its characters. (See pi. xviii, figs. 1, 12, 13.) In the most 

 ancient allied types, we should, therefore, look for thin, delicate, 

 ovate shells, without any differentiated hinge-teeth. A small in- 

 ternal cartilage, or resilium, was probably coexistent with an ex- 

 ternal ligament not differentiated from the general perisostracum, 

 and connected directly with the resilium. The latter is formed by 

 the invaginated cells of the primitive shell-gland of the veliger, and 

 the perisostracum by the cells of the general ectodermic membrane, 

 which is continuous with the shell-gland at first ; therefore, the two 

 structures must have been continuous, primarily, and probably of 

 simultaneous formation. The greater part of the early bivalves 

 have the resilium' and ligament both developed, but in many of the 

 more modern genera, and also in some palaeozoic genera, one or the 



* Dr. R. T. Jackson, Phylogeny of the Pelecypoda, Mem. Boston See. Nat. Hist., 

 iv. No. 8, p. 277. 1890. 



•2 Science, vol. v, p. 166, Jan. 29, lh97. 



^ The useful term resilum was proposed by Dr. Dall, for the so-called internal car- 

 tilage of the hmge of bivalves; resilial pit may replace ''cartilage pit." 



