(ii.) TELEOSTOMI; _e.g_. such well-known Fishes as the Perch, Cod,{149} Salmon, and Herring, and also the less familiar "Ganoids," living and extinct. (1) Crossopterygii; e.g. _Polypterus_. (2) Chondrostei; _e.g_. the Sturgeons (_Acipenser_). (3) Holostei; _e.g_. the Bow-fin (_Amia_), and the Gar Pike (_Lepidosteus_). (4) Teleostei; _e.g_. the Perch, Cod, Salmon, etc. (iii.) DIPNOI; e.g. _Neoceratodus, Protopterus_, and _Lepidosiren_. _Appendix to the Class Pisces_. (i.) PALAEOSPONDYLIDAE†; e.g. _Palaeospondylus_. (ii.) OSTRACODERMI†. (1) Heterostraci; e.g. _Pteraspis_. (2) Osteostraci; e.g. _Cephalaspis_. (3) Anaspida; e.g. _Birkenia_. (iii.) ANTIARCHI†; e.g. _Pterichthys_. (iv.) ARTHRODIRA†; e.g. _Coccosteus, Dinichthys_. † Entirely extinct. The Fishes included in the Teleostomi were formerly arranged in two groups: the Ganoidei, including the Crossopterygii, Chondrostei, and the Holostei, with their numerous fossil allies; and the Teleostei. Living Ganoids agree with one another, and differ from Teleosts in possessing an intestinal spiral valve and a conus arteriosus. It is difficult, however, to separate the two groups, inasmuch as in each group there are living forms which tend to approximate to the other; and numerous fossil genera, of whose soft parts nothing is known, are in many respects intermediate between the two. The position and relationships of the Palaeospondylidae, Ostracodermi, Antiarchi, and Arthrodira are very uncertain. The Palaeospondylidae have been included in the Cyclostomata, or at all events have been regarded as more or less closely related to that group, while the absence of paired fins and the apparent want of jaws have suggested that the Ostracodermi occupy an intermediate position between the Cyclostomata and the Gnathostomata.[120] On the other hand, the Arthrodira are either regarded as an independent group of Fishes, or are included amongst the Dipnoi. In the latter case, the Dipnoi are divided into the Arthrodira and the Sirenoidei, the last mentioned group including _Neoceratodus, Protopterus_ and _Lepidosiren_, and their extinct allies. {150}CHAPTER VI EXTERNAL CHARACTERS OF CYCLOSTOMATA AND OF FISHES EXTERNAL CHARACTERS—COLORATION—POISON GLANDS AND POISON SPINES— PHOSPHORESCENT ORGANS. [Illustration: FIG. 91.—_Petromyzon marinus_. A, ventral; B, lateral; and C, dorsal, view of the head. _br.cl.1_, First branchial cleft; _buc.f_, buccal funnel; _eye_, the eye; _mth_, mouth; _na.ap_, nasal aperture; _p_, papillae; _pn_, pineal area; _t^1_, _t^2_, _t^3_, teeth of buccal funnel; _t^4_, teeth on the tongue. (From Parker and Haswell, after W. K. Parker.)]