AZYGOGRAPTUH. 93 



fore, further abortion might lead to the non-development of the api)arently funetioii- 

 less crossing canal : and this seems to be precisely what we have in Azygograjjtus. 



The Azygog-rapti fall into three groups, according to the characters of the stipe 

 and the tlieca3 ; these groups are as follows : 



Group I. Type A::. Lnpujorthi. 



Az. Lainrortlii. 

 Az. HicJcsii. 

 Group II. Type Az. suecicus. 



Az. saecicns. 

 Group III. Type Az. cwlrhs. 



Az. ccelehs. 



Group I. — Type Az. Lapwortld. 

 Azygograpti in which the dorsal wall of the stipe has a graceful convex 

 curvature ; the stipe originates at a point on the sicula midway between the apex 

 and the aperture, and grows at once outward ; the tliecae are large, and the 

 amount of ovei-lap is small. 



Azygograptus Lapworthi, Nicholson. Plate XIII, figs. 1 a, h. 



1875. Azijgograptus Lapworthi, Nicliolsou, Auu. Mag. Nat. Hist. [4], vol. xvi, p. 2G9, pi. vii, figs. 2, 2 c. 

 1898. Azygograjdus Lapworthi, EUes, Quart. Jouru. Greol. Soc, vol. liv, p. 513. 



Stipe considerably curved, 2'5 to 5 cm. in length, slender at origin, but 



gradually increasing up to a maximum Avidtli of about 1 mm. ; originating 



from a conspicuous sicula at a point about midway between the apex and 



aperture. Tliecge seven to eight in 10 mm., long and narrow, four to four 



and a half times as long as Avide, and widening towards their apertures; 



outer walls straight, or with slight concave curvature near the sicula; 



inclination 20", in contact only. Apertural margins straight, perpendicular. 



Descriptiun. — The stipe is always convexly curved ; it is slender at its origin, 



the width opposite the aperture of the first theca rarely exceeding "5 mm., but it 



increases steadily till it I'eaches the maximum of about 1 mm. 



The sicula is large and conspicuous ; when coni- 

 Nicholson. plete it measures fully I'o mm. m length (iNicholson s 



measurement is somewhat less), and is always com- 

 pletely free in its apertural region. Occasionally 

 transverse rings may be seen very near the apical 

 end. 

 Eniai-K-cmont of one of Tlic first thcca ai)i)ears to yrow out from the 



ou PI. XIII, fi^. la. . . ' ^ -^ 



sicula at a point midway between the apex and the 

 aperture, and curves away at once ; hence the connection between the stipe and the 



