1S4 MATTETEW ON CAMBEIAN 



the markings are more distinct near the margin than toward the umbo, while the reverse 

 is the case with this Obolus. 



This larval part of the shell is sometimes divided by one or two concentric lines into 

 two parts, of which the outer is distinguished by au enlarged pattern of the radular 

 ornamentation ; and the margin of this zone has a more rounded outer margin than the 

 inner zone, owing to the more rapid extension of the margin at the sides and front. In 

 this outer half of the larval zone there is a more decided thickening of the shell, for the 

 lines of growth are not so delicately marked as in the inner half, nor is the impression of 

 the features of the interior of the shell so clearly apparent, except as regards the outlines 

 of the visceral cavity. These features, however, may be inferred from the rounded ridges 

 on the surface and by the lines of scars left by the anterior adjustor muscles, which give 

 evidence of about six stages of growth in this oirter zone of the larval shell. 



Outside of this zone the radular ornamentation is exchanged, on a narrow band of the 

 shell, for concentric ridges, indicating an arrest of growth preparatory to the next phase 

 in the life history of the indiAdduals of this species. 



The close of the larval period is marked by the fixation of the hinge line, which no 

 longer lengthens, and consequently the jiosition of the posterior adductor and the 

 proximate end of the cardinal muscle do not materially change after this. 



(3.) The advent of the next phase in the history of this shell, which may be called the 

 adolescent phase, is indicated by a return to the radular ornamentation, which now is of 

 a still coarser pattern than previously, and is not always well preserved ; in fact dorsal 

 valves are not uncommon, and ventral valves are occasionally found, which show no 

 radular ornament at this period, birt have concentric lines only. This peculiarity, how- 

 ever, may be due to imperfect preservation. 



This part of the valve, like the larval, is not unfrequently found to be divided into 

 two zones by a few concentric lines ; in the outer of these zones the radular ornament is 

 usually very irregular. 



At this period one does not find the lines of growth so distinctly marked as in the 

 earlier period, nor the scars of the anterior adjustor and adductor muscles ; but the out- 

 line of the visceral cavity, owing to the thickening of the callus formed there, stand out 

 with greater distinctness (fig. 2e.) 



The adult shell presents, in both valves, a nearly round contour, and on it also is 

 rather prominently indicated the outline of the visceral caAuty, which becomes propor- 

 tionately narrower in the adolescent and adult stages, than at an earlier period. The 

 marks of the muscles in the previsceral area of the shell also continue to be faintly visible 

 on the outer i^ortion of the shell. 



This species, then, is marked by four stages of growth and development, of which 

 the most prominent features are the following : — 



(1.) Embryonic. — Formation of the embryonic shell. 



(2.) Larval. — Lengthening of the hinge line and acquisition of mantle-margins. 



(3.) Adolescent. — Fixation of the hinge line, otherwise as the last, except that the 

 radular ornament becomes irregular. 



(4.) Adult. — Absence of radular ornamentation on the valve, and great exj)ansion of 

 the mantle margin. 



The species of brachiopod which most nearly approaches this is Lingula ? (or Obolus ?) 



