4S0 AXEL A. OLSSON 



row, impressed line or sulcus into two unequal areas, expressed internally as 

 a weak or stout rib ending at the margin as a short spur or tooth. The 

 anterior area of the disk, generally larger, is sculptured with crenulated 

 concentrics ending as radial ribs at the anterior end; the more posterior 

 part with concentrics only, the posterior end of the right valve produced 

 in the shape of a short tongue with smooth or pectinate margins. Apophj'ses 

 short, styloid, or shaped like a small chondrophore and is developed prin- 

 cipally in the left valve. In Jouannetia, s.s., there is a large, platelike lamina 

 or septum which stands vertical in the cavity of each valve and served for 

 the attachment of the posterior adductor muscle; this septum is absent in 

 Pholadopsis. Borers in rock or coral. 



Divided into two subgenera as follows: 



I. Umbonal cavity in each valve bearing a large, platelike lamina or 

 septum (myophore) for the attachment of the posterior adductor 

 muscle. Margin of the siphonoplax developed at the posterior end of 

 the right valve, smooth or entire. 



Subgenus Jouannetia, s.s. 



II. No platelike myophore. Siphonoplax with dentate or serrate margins. 



Subgenus Pholadopsis 



Jouannetia (Jouannetia) duchassaingi Fischer Plate 80, figures 1-lc 



Jouannetia duchassaingi Fisciier, 1862, Jour, de Concliyl., vol. 10, pp. 375-376, pi. IS, 



fig. 3. 



Jouannetia {Jouannetia) duchassaingi Fischer, Turner, 1955, Johnsonia, vol. 3, No. 34, 



pp. 135, 136, pi. 82. 



The adult shell is large, two inches or more in length, ovate, globose 

 when complete but at present, the species is known from a few, isolated 

 valves only. The anterior gap is closed by a large, convex callum which is 

 continued above to form a separate lobe which covers the sides of the um- 

 bone. Valves unequal, each one divided mesially by a narrow, sharp sulcus 

 into two segments; the anterior segment has a nearly straight edge and is 

 neatly marked with fine, crenulated concentrics and on its most anterior 

 portion by radial riblets, heavier forward; the posterior segment is sculptured 

 more plainly with concentrics and resemble coarse, growth incremental. 

 The right valve is larger than the left, its posterior end extended into a 

 short, tongue-like, thickened lobe which projects a little beyond the margin 

 of the left valve; this lobe has plain margins. The myophore in the interior 

 of the umbonal cavity is a flat plate, standing vertical and it is attached to 

 the shell wall just behind the rib formed by the external sulcus. 



Fischer's type specimen was collected in Panama, its exact station not 

 known. The Manta record is based on three, incomplete, beach valves. J. 

 cumingii of the Indo-Pacific is related but a larger species. The other known 

 species are fossil in Europe; all are smaller. 



Range — Ecuador to Panama. Panama: Panama (Fischer). Ecuador: 

 Manta. 



Subgenus PHOLADOPSIS Conrad, 1849 



(Triomphalia Sowerby, 1849). 



Type species by monotypy, P. pectinata Conrad. Eastern Pacific. 



Without an internal platelike myophore. Posterior margins of the right 

 valve serrated. 



