198 



BRITISH GRAPTOLITES. 



perpendicular free wall ; apertural margin horizontal, situated in semi- 

 circular excavations occupying one-fourth the width of the polypary and 

 one-fourth of the ventral margin. 

 Description. — The polypary often attains the considerable length of 5 or 6 cm. 

 or more ; it is usually 1 nun. wide at its origin, but may increase gradually up to a 



maximum breadth of 2*5 mm. ; this is 



Fias. 128 a and b.- 



m 



W 



- 



a. Specimen showing proximal end in relief, obverse 

 view. Note large sieula and origin of sac. Dol)b's 

 Linn, Lower Hartfell Shales. Coll. Wood. 



I. Reverse view, showing spines on basal thecoe ; sac 

 distorted. Ibid. Coll. Elles. 



Climacograptus Wiisoni, Lapw. only rarely attained in the largest speci- 

 mens, an average breadth of 2 mm. being 

 of commoner occurrence. The sides of 

 the polypary are subparallel, except at 

 the proximal end. In the obverse aspect of 

 the adult the sieula is visible for 2"5 of 

 its length, and is entirely free in its 

 apertural region for fully 1 mm ; the 

 apical region tends to be more or less 

 completely concealed by the growth of 

 th. 3 1 and th. 3' 3 . Th. I 1 originates from 

 the sieula fully 2 mm. above its aperture, 

 and grows at first downward, partly em- 

 bracing the left side of the sieula; it then 

 bends abruptly round and grows outward 

 and upward for a distance of about 

 0-5 mm. A spinous outgrowth from the thecal wall of th. I 1 below its aperture may 

 also occasionally be seen. Th. I 2 shows a similar outgrowth. The apertural part 

 of th. .I s is all that is visible in this aspect, since its initial portion (crossing canal) 



is concealed by the sieula. Th. 2 1 seems to develop 

 from the back of th. I 2 and grow behind the sieula; 

 th. 2 3 grows similarly from the back of th. 2 1 ; and 

 th. 3 ] appears to grow from the front of th. 2 : , 

 thereby concealing part of the apex of the sieula. 

 At tins stage the septum appears; thus it happens 

 that in the fully developed polypary the apex of 

 the sieula is concealed, although in younger stages 

 of growth it is as a rule completely visible. 



Tii the reverse aspect the alternate development 



of the earliest thecse can be clearly made out. The 



early growth of th. I 2 is approximately horizontal 



in direction; but ultimately it grows outward and 



upward. Th. 2 1 is also horizontal at first, but th. 2 2 grows at once outward and 



upward. The septum now appeal's, and in subsequently developed thecee the growth 



is first upward, then slightly out ward, and finally again upward. 



Pig. 128 c. — Climacograptus Wil 

 Lapw. 





Distal fchecae. Dobb's Linn, Low< 

 Hartfell Shales. Coll. Wood. 



