LIVING BRACHIOPODA. 323 



disk (42: 9), near the posterior margin, an irregular nucleus appears, surrounded by a few 

 eccentric lines. It is an interesting fact that the successive stages of Discinisca begin 

 with a nucleus eccentrically related to a perfectly circular stage ; this circular stage, in 

 turn, being eccentrically related to an oblong-ovate stage, which is the true protegulum ; 

 and this, in turn, being eccentrically related to an adult stage which is finally orbicu- 

 lar again. 



The above description applies to the dorsal shell. I shall leave the relations of these 

 stages to the early Cambrian forms to Dr. Beecher and Mr. Schuchert who have done so 

 much to the elucidation of these interesting problems. The ventral shell does not show 

 this circular disk, but a bi-circular outline arching inward on its posterior border. A 

 thickened ridge runs through this stage in a median line to merge into a thickened ante- 

 rior border (42: 13) . The brachia, or rather the brachial cirri, start from the thickened, 

 anterior border, as may be seen by referring to 61 : 9. The bi-circular outline of the ven- 

 tral shell corresponds to the circular outline seen in the dorsal shell above described. A 

 greatly magnified view of the ventral shell is shown in 42: 10. A wide peduncular notch 

 is formed by the rapid growth backward of the shell; on the sides of this wide notch, 

 another growth appears with a new notch much narrower. In tins figure, the beginning 

 of the median septum appears, on each side of which a polygonal shell structure is seen. 

 The anterior border shows three concentric outlines, and the rim is marked by irregular 

 radiating lines. It will be seen that the long deciduous setae start from this primitive 

 bi-circular outline (42 : 11). 



The shell of D. stella is quite different from that of D. lamellosa. When I first 

 dredged this species at Nagasaki, I mistook it for a species of Crania. The dorsal shell is 

 quite firm and very irregular in shape. The creature clung so closely to the rock, when 

 first observed, that not until it erected itself on its peduncle did I become aware that it 

 was a species of Discinisca. In D. lamellosa the peduncle issues very close to the poste- 

 rior margin of the ventral shell, the shell substance behind the peduncle being membran- 

 ous ; in D. stella the peduncular opening is nearer the centre of the ventral shell, the 

 dorsal shell is irregularly convex and laps considerably over the ventral shell, which is of 

 extreme tenuity. It appears almost membranous and is so intimately blended with the 

 ventral pallium that it must be considered an organic part of it. The shape of the ventral 

 shell may be compared to a shallow pan with flaring rim and with the bottom pushed 

 upwards. At 42 : 17 are shown the dorsal and ventral shells with peduncle in outline, 

 while at 17is shown the lower valve turned upside down; a series of radiating lines, some 

 of which are branched, runs from the peduncular opening nearly to the periphery, turning 

 up on the rim which meets the dorsal shell. These lines are somewhat irregular and 

 clusters of fine striae occur at intervals along them (42: 15). The markings of the 



