62 THE NAUTILI] S. 



opisthodetic but in front of the beaks the periostracum is produced 

 inside the dorsal margin as well as externally. This interior exten- 

 sion covers a narrow strip of the thickened dorsal margin of each of 

 the valves, leaving about two-thirds of these pseudo-nymphs bare and 

 strongly radially grooved and striate, the radii diverging from the 

 dorsal margin of the valves slightly in front of the beaks, and doubt- 

 less serving to make more eflScient the local attachment of the 

 periostracum, which is here thickened and expanded. The ligament 

 is wide and strong, external, but visible in the gap between the 

 valves behind the beaks and supported by heavily calcified nymphs. 

 Behind the nymphs the dorsal margin of the shell on each side ex- 

 hibits a long and conspicuous indentation. The valves are heavily 

 calcified, internally radiately striate, the ventral margin straight, the 

 dorsal margin nearly parallel to it except as modified by the nymphs, 

 the two valves touching only near the beaks, which are low but 

 swollen, the rather narrow dorsal interval betw^een the valves being 

 covered by a continuous sheet of the thick periostracum. The 

 posterior muscular impression is of moderate size and obliquely 

 ovate, the anterior smaller, narrow and rhomboid in shape ; the pal- 

 lial line is obscure and continuous, situated close to the margin of 

 the valves; externally the surface is smooth except for lines of 

 growth and a number of shallow, wide, radiating channels which 

 proceed from the beaks toward the margin of the valves, where they 

 produce a certain amount of undulation. Anteriorly there are eight, 

 posteriorly six of these channels, with a median space which has no 

 rays, and, on the basal margin of the valves, is about 50 mm. wide. 

 The beaks are about 75.0 mm., in front of the posterior end. The 

 shelly part of the valves is 191 mm. long (the total length including 

 periostracum is about 240 mm.), the height 62 mm. (with periostra- 

 cum about 100 mm.), and the estimated diameter of the valves in 

 life about 60 mm. 



The perfect condition of this specimen enables us to understand 

 the origin and use of the striated and thickened area of the an- 

 terior dorsal margin of the values, already noted by me in S. (A.) 

 agassizii from the Gulf of Panama. To preserve it in its present 

 satisfactory state it will be kept in alcohol. 



"With this remarkable specimen was obtained an interesting shell 

 of Vesicomya, also without the soft parts, which may be described as 

 follows : 



