TRANSACTIONS OF SECTION D. 4.2] 
showing specialisation to a remarkable degree. Out of thirty recognised genera 
of African Cichlids, as many as fifteen are believed to be peculiar to Tanganyika. 
Lake Nyassa, with tle Upper Shiré, possesses also some remarkable endemic 
genera; but they are only four in number, and the number of species recorded up 
to the present does not exceed twenty-two. The rest of the species are mostly 
from West Africa and the Congo basin; but afew, referable to the two most 
widely spread genera, are found in East and South Africa. Madagascar has only 
four species, two belonging to an endemic genus, whilst each of the two others 
is referred to a widely distributed African and Syrian genus. 
No fossils are known that agree closely with any of the recent genera, but a 
type of Perciforms, described by Cope as Priscacara, from Middle Kocene fresh- 
water beds in North America, presents all the characters which we should 
expect to find in the direct ancestors of the modern Cichlids, differing from the 
living forms in the presence of vomerine teeth, a serrated pre-operculum, and 
apparently eight branchiostegal rays. It has twenty-four vertebre, a number 
Jower than is found in most of the recent genera ; and this indication is of import- 
ance for reasons that must be explained somewhat fully. 
The lower Teleosteans (Malacopterygii and Ostariophysi, often united under 
the term ‘ Physostomi’) mostly have a high number of vertebree; but when we 
pass on to the higher Acanthopterygii, we find very frequently, among most 
diverse families, the number reduced to twenty-four. That this number should 
occur with such frequency has struck many ichthyologists since Dr. Giinther 
first drew attention to it, over forty years ago, pointing out at the same time 
that in the Labride this number is almost constant in the tropical genera, whilst 
those genera which are chiefly confined to the temperate seas of the northern and 
southern hemispheres have an increased number. It has since been shown by 
Dr. Gill and by Professor Jordan that this generalisation holds true of several 
other families of Acanthopterygians, and the latter authority, when discussing the 
subject at some length, came to the opinion that the state of things could be ex- 
plained, from an evolutionary point of view, on the assumption that competition 
among various marine fishes being greater within the tropics has resulted in 
greater specialisation, by which the originally high number of vertebree has been 
reduced. It is difficult, however, on this assumption to account for the fact that 
in so many cases the reduction should have resulted in the number twenty-four— 
neither one more nor one less—and this repeated in many families belonging to the 
same sub-order but otherwise only remotely related to one another. Three years 
ago, when dealing with the affinities of the Flat-fishes, Plewronectide, 1 was 
struck by the discovery that, in the unquestionably least specialised genus, 
Psettodes, the vertebre are twenty-four in number, the other known genera 
having from twenty-eight to sixty-five, and that the numbers increased along the 
most probable lines of evolution. A consideration of other families, and of the 
fossil forms in which the number of vertebree has been ascertained, soon con- 
vinced me that this rule also applies to them, and that the order of evolution had 
in every case to be reversed from that assumed by Professor Jordan, whose inter- 
pretation I had previously accepted as correct. Asa result of my investigation into 
this question I believe that the frequent occurrence of twenty-four vertebre is due 
to the original Acanthopterygians having presented this number, that it has been 
retained in the more generalised members of the families which have branched off 
from them, and increased or, more seldom, reduced in the course of evolution. 
This opinion is supported by the available paleontological data, several of the 
Cretaceous Berycide (Beryx, Hoplopteryx), the earliest Acanthopterygians, 
having twenty-four vertebrae, and the same number occurs in the Cretaceous 
Scorpidide ; Prolates, the earliest true Perciform, from the lowermost Eocene 
beds, formerly ranked as Upper Cretaceous, has the same number. Several other 
families afford good support to this theory: In the Echeneidide, the Eocene 
Opisthomyzon has twenty-four vertebre, the living Echeneis twenty-seven to 
thirty ; again, the earliest known Cottid, the Eocene Hocottus, has twenty-four, the 
Oligocene-Miocene Lepidocottus twenty-six, whilst the living members of this large 
family haye no fewer than thirty; the earliest known Blenniid, Pteryg ocephalus, 
