22 Prof. H. A. Nicholson—Organisms in Paleozoic Limestones. 
Solenopora, as it differs from the type of the genus in important 
structural characters. This is more particularly seen in the regularly 
prismatic form of the component tubes of the skeleton, and in the 
total absence of the septa-like process produced in Solenopora 
compacta by the commencing fission of the tubes. At the same time, 
it agrees entirely with S. compacta in its general form and mode 
of occurrence, and especially in its being composed of imperforate, 
tabulate tubes of excessive minuteness. Upon the whole, therefore, 
it seems safest to place it temporarily in the genus Solenopora. 
Fie. 4.—A, Tangential section of Solenopora ? filiformis, Nich., from the Ordovician 
limestone of Craighead, Girvan, enlarged about 60 times. B, Vertical section of 
the same, similarly enlarged. 
It cannot be said that the present species throws much fresh light 
upon the systematic position of the genus Solenopora. If viewed 
without reference to the size of its tubes, it might quite well be 
regarded as a Monticuliporoid, and might be placed. under the genus 
Monotrypa. The extraordinary minuteness of the tubes would seem, 
however, of itself, sufficient to preclude a reference of this fossil 
to the Monticuliporoids. If, however, we admit that the genus 
Mitcheldeania may be referred to the Hydrozoa, we can get over one 
of the principal difficulties attending the supposition that the genus 
Solenopora is referable to the same great class—the difficulty, namely, 
that no known Ceelenterate possesses a skeleton of such an excessively 
minute character. Upon the whole, therefore, I am inclined to think 
it may be tolerably safe to regard Solenopora, Dyb., as representing 
a peculiar extinct group of Hydrozoa, though I do not think that 
the evidence upon this point is in any way conclusive. 
Genus GirvaNeLLA, Nicholson and Etheridge, Jun., 1880. Fig 5. 
Largely concerned in the formation of the Ordovician limestones 
of Ayrshire, and commonly associated with the preceding, is another 
remarkable organism which was described in 1880 by Mr. R. 
Ktheridge, jun., and myself, under the new generic and specific name 
of Girvanella problematica (Mon. Sil. Foss. Girvan, p. 28, pl. ix. 
stone at Craighead, sometimes being the principal agent in the 
fig. 24). This curious fossil occurs in great numbers in the lime- 
formation of the rock, and presents itself in the form of small 
