152 MATTHEW ON CAMBEIAN 



branches of the cardinal muscle; outside of these two ridges are a pair of elongated 

 semilunar scars, where the i^osterior adjustor muscles were attached. There is a -shaped 

 ridge flatter than those described aud broader, in front of the space between the forks of 

 the mesian ridge ; this probably divides the anterior adductor muscles. 



Interior of the Ventral Valve (Figs. 2 h to I). — There is much resemblance in general aspect 

 between the interior of the dorsal and ventral valves. The latter differs in the more elon- 

 gated callus of the visceral cavity, the narrower scar of the posterior adjustor muscle, the 

 absencr^ of the strong lateral ridge beside this muscle, and the want of a division along 

 the mesian line. This valve also exhibits indications of the attachment of the pedicel, and 

 of the central adjustor muscles. The ventral valve also possesses a smaller, pointed depres- 

 sion in the front of the visceral cavity, which probably marks the attachment of the anterior 

 adductor muscle. From this point, a somewhat depressed band extends to the front of the 

 shell. If we assume that the small round scar, near the centre of the visceral cavity, was 

 made by the central adjustor muscles, a small scar behind it, having its opening directed 

 backward, will mark the starting point of the pedicel. This organ in its backward coiirse 

 has made a slight groove on the axial line. There are indications that the pedicel then 

 passed through a foramen in the hinge, coming on the hinge area below the beak, but this 

 point is not clearly determinable. 



Sculpture. — The younger part of the shell is covered with minute tubercles, sloping 

 forward, and arranged in curved rows which arch forward to the mesian line from each 

 lateral margin of the valve, thus giving the surface a cancellated appearance ; this cancel- 

 lated or rasp-like surface does not cover the valve continuously, but is interrupted by 

 arching bands of ridges concentric to the umbo. The anterior part and the outer lateral 

 parts of the valves have the concentric ridges only, with a few faint, broken, radiating lines, 

 visible at intervals. The cancellated lines do not always cover so large a space on the 

 dorsal as on the ventral valve, but the former valve shows more distinctly the radiating 

 lines outside the visceral cavity. 



Growth and Development. — The growth and development of this species as recorded in 

 its shelly covering are very instructive. Beginning with a shell which is comparatively 

 tumid in form and nearly semicircular in outline, it finally becomes orbicular in outline, 

 and with valves flattened to the form of saucers. 



By the varied sculpture, the outlines of the valves and the surface markings, several 

 phases in the life of this brachiopod may be distinguished. 



(1.) The first is that marked by the embryonic shell. This shell, now preserved in the 

 umbo of the adult, shows in the markings on its surface faint indications of additions to 

 its size, but these are hardly discernible. A remarkable feature about this is the form 

 which is entirely different from that of the adult, for it (in the dorsal valve especially) is 

 nearly semicircular in outline, and is quite tumid when compared with the adult shell ; 

 it looks more like an Orthis or a Linnarssonia (see figs. \a to c) than an Obolus or a 

 Lingulella, the two genera which the adult most nearly resembles. The embryo ventral 

 valve also differs quite as much from the adult as does the dorsal, for in its high umbo 

 and straight hinge line it recalls species of. the genera Acrotreta and Kutorgina (see figs 

 2« to c.) 



In the embryonic shell of the dorsal valve, which is narrowly semicircular, the 

 straight outline of the hinge was scarcely broken by the rounded projection of the umbo. 



