Vol. 56.] FORMATION AND ITS GRAPTOLITE-FAUNA. 451 



and, as we are now aware, frequently incorrect. Consequently 

 the graptolite-fauna of the formation stands greatly in need of 

 revision. 



The graptolitic species of the Lower Ludlow Beds show many 

 resemblances among themselves, and the graptolite-fauna as a whole 

 presents certain peculiarities of its own. In the first place, the 

 polypary, in the large majority of the forms, is straight for the 

 greater part of its length, but is distinctly curved inward at the 

 proximal extremity. This shape of the polypary is almost peculiar 

 to the Lower Ludlow graptolites, and stands in marked contrast to 

 that of the Wenlock-Shales graptolites, which as a rule is curved 

 outward proximally. 



Secondly, the shape of the thecae is strangely similar in most Lower 

 Ludlow species. The apertures are either spinose, or they are wholly 

 destitute of ornamentation. The development of the apertural spines 

 appears to have been gradual and also subject to variations, for not 

 only is there every gradation between species which possess only a 

 single apertural spine and those that have all their thecae spinose, 

 but even in the same species the number of spines is variable. The 

 development of spines would seem to be the expression of an instinct 

 of preservation, and it is interesting to note that the later forms of 

 trilobites show similar characters. This leads one to infer that 

 the extermination of both graptolites and trilobites may have been 

 due not merely to unfavourable physical conditions but also to the 

 existence of powerful enemies. 



In spite, however, of the general similarity in form of the 

 Lower Ludlow graptolites, a further and more complete examination 

 of the fauna reveals the existence of a large number of species and 

 varieties. The separation of these is difficult, for as the number of 

 specimens examined is increased, the more is one induced to accept 

 the views of those who assert that species and varieties merge the 

 one into the other by almost imperceptible gradations. It would 

 be useless for the purposes of this paper to describe all the various 

 transitional forms, and I shall therefore confine myself to describing 

 and naming (a) those which are of zonal value in working out the 

 stratigraphy of the Lower Ludlow Beds ; and (b) those of special 

 palaeontological interest. 



As regards the classification of the species and varieties, it is 

 found to be most convenient to group them round special types. 

 The character which I consider to be of most value for the purposes of 

 such grouping is undoubtedly that of the proximal extremity, for it 

 is more constant than any other and appears to be less liable 

 to variations. In the present general state of our knowledge, 

 however, other characters, such as the form of the polypary and 

 the shape of the thecae, must still in many cases be the main 

 guide in classification, for we must assume provisionally that the 

 general morphology of the form is the best index of phylogenetic 

 relationship. 



The Lower Ludlow graptolite-fauna includes two families and two 

 genera, namely Monograptus and Metiolites, while two families and 

 three genera are contained in that of the Wenlock Shales below. 



