Distribution of the RJtahdophora. 27 



shown, marks the basal beds of the Wenlock of Siluria, Scan- 

 dinavia, and probably also of the less carefully studied areas 

 in Central Europe. 



Family ii. Leptograptidse. 



Like the previous family, that of the Leptograptidee appears 

 to have had but a very limited range in Palaeozoic time. 

 None of its species is known to occur in the Cambrian or 

 Silui'ian. The family is exclusively Ordovician in its range, 

 its oldest genera appearing in the Upper Arenig, and its 

 newest forms vanishing within the limits of the Upper 

 Caradoc. 



As a whole, the family is especially characteristic of the 

 Bala formation, numerous species pervading it from its base 

 almost to its summit, while the individuals are distributed 

 with tolerable equality throughout the entire succession. 



The two sections into which the family is provisionally 

 divided attain their maxima along widely separated horizons. 

 The species composing the oldest section, which includes 

 Ccenograptus, Hall, and its allies, culminate and disappear 

 within the limits of the Llandeilo-Bala (Glenkiln) subforma- 

 tion. The species which compose the second section (which 

 includes Leptograptus, Ampfrigraptus, &c), on the other 

 hand, are comparatively rare in the Llandeilo-Bala. They 

 show a rapid increase in numbers and importance as we ascend 

 into the Bala proper ; and about the horizon of the Bala Lime- 

 stone they become, in the Hartfell shales of the south of 

 Scotland, most abundant and characteristic. Here, however, 

 we suddenly lose sight of them; and not a fragment of a 

 species of the Leptograptidas has yet been detected in the 

 higher zone of the Bala or in any more recent formation. 



Coenograptus. — The beautiful genus Coenograptus of Hall, 

 which we regard as the type of the older section of this family, 

 is unknown outside the limits of the Llandeilo-Bala formation. 

 Of the zone bounded below by the Llandeilo Limestone, and 

 above by the deeper beds of the Caradoc proper, this genus is 

 the peculiar and characteristic fossil. The form Coenograptus 

 gracilis of Hall marks these beds everywhere in Britain ; and 

 this fixes the systematic position of the Coenograptus-bearmg 

 strata of Sweden, New York, and Australia. Along this zone 

 the horizontal distribution of this form appears to have been 

 world-wide. 



Azygograptus. — By far the most remarkable genus at present 

 assigned to this division is the strange form Azygograptus 

 (Nich. & Lapw.). It may possibly be shown to belong to the 

 I)ichograptida3. In that event the range of the Leptograptida> 



