174 



HARDWICKE'S SCIENCE-GOSSIP. 



According to Dr. Albert Giinther,* the living 

 Plagiostomi may be divided into fifteen families, — 

 Selachoidei nine, and Batoidei six, — and, so far as can 

 be ascertained, all but one are represented in the fossil 

 state. Seven others have already been established 

 for curious extinct groups, and, as palasontological 

 research progresses, it is not in the least improbable 

 that the families will be still further multiplied. 



Following Dr. Giinther's arrangement, for the 

 most part, and including the results of recent investi- 

 gations, we may place the different fossil genera in 

 their respective families, as in the table below. It is 

 always difficult to determine precisely the affinities of 

 the fragmentary relics preserved in nature's stony 

 record, and hence this classification cannot be regarded 

 as more than provisional, although it is perhaps the 

 best that the present state of science will allow. 

 Only the genera that are now quite extinct are 

 printed in italics, and more attention will be given to 

 these in the sequel than to those which still exist and 

 inhabit recent seas. 



LIST OF FAMILIES OF PLAGIOSTOMI 

 (with their probable fossil representatives). 



1. Carchariid.'E. [Dorsal fin-spines absent; teeth, in most 

 genera, conical or triangular, and laniary.] Corax, Galeo- 

 eerdo, Hcmipristis, Zygaena (Sphyrna). Range, Cretaceous — 

 Recent. 



2. Lamnid.e. [Mostly pelagic' fishes of large size. Dorsal 

 fin-spines absent; teeth conical or triangular, and laniary.] 

 Lamna, Odontaspis, Oiodus, Oxyrhina, Carcharodon, Sphc- 

 nodus. Range, Jurassic — Recent. 



3. RHINODONTID.E. [No fossil forms known.] 



'' 4. NOTIDANID.E. [Only one genus, Notidanus, living. 

 IDorsal fin-spine absent ; teeth laniary.] Notidanus. Range, 

 Jtirassic — Recent. 



5. ScYLLiiD.E. ["Dog-fishes." Dorsal fin-spines absent; 

 teeth very small, and more than one row at a time in use.] 

 Thyellina, ScylUodtis, Scyllium. Range, Cretaceous — Recent. 



6. HvBODON'TiD/E. [An extinct family.] Ctoiacatithus 

 [Cladodus], Tristychiiis, Pristicladodus, Hyhodus. Range, 

 Carboniferous Limestone — Chalk. 



7. Orodontid^e. [An extinct family.] Orodus, Lophodus. 

 Range, Carboniferous. 



8. CocHLiODONTiD.E. [An extinct family.] Psephodus, 

 Cochliodus, Streblodiis, Deltodus, Sandalodus, Toinodus, Pocci- 

 lodus, Pleurodiis. Range, Carboniferous. 



9. CopodonTID/E. [An extinct group, possibly a family.] 

 Copodus, Labodus, Phiacodus. Range, Lower Carboniferous. 



10. PsAMMODONTiD.'E. [An extinct group, possibly a family.] 

 Psamineius. Range, Lower Carboniferous. 



11. CESTRACiONTiDyE. [Only One genus living, — the " Port 

 Jackson Shark" of Australian coasts. Dorsal fin-spines present 

 in Cestracion and Acrodus ; teeth obtuse, adapted for crush- 

 ing.] Acrodus, Strophodus, Ptychodus. Range, Trias — 

 Recent. 



12. Spinacid.1!. Two dorsal fin-spines generally present ; 

 teeth numerous, and very variable in form.] Palceospmax, 

 Drepanephoms, Spinax. Range, Lias — Recent. 



13. Rhinid.'E. ["Angel-fishes" and "Monk-fishes." This 

 family, and the three following, may be regarded as intermediate 

 between the Selachoidei proper and the true Batoidei.] T/utu- 

 nias, Squatina. Range, Jurassic — Recent. 



14. Xenacanthid.1£. [An extinct family.] Plcuracanthus. 

 Range, Carboniferous — Permian. 



15. Petalodontid.«. [An extinct family.] Ctenoptychius, 

 Petalodus, Petalorhynchis, Polyrhizodus, Jajiassa. Range, 

 Carboniferous — Permian. 



16. Pristiophorid>e. [Snout produced into flat rostrum, 

 ganerally with lateral teeth.] Sqicaloraja. Range, Lias — 

 Recent. 



17. Pkistid.!!. [" Saw-fishes." Toothed rostrum propor- 

 tionally larger and better developed than in the last family.] 

 Pristis. Range, Eocene — Recent. 



* "Catal. Fishes Brit. I\Ius.," vol. viii. {1870). See also 

 Giinther's " Study of Fishes," pp. 312-348. 



18. RniNOBATiDyE. [Body not remarkably depressed, tail 

 long. Teeth numerous and obtuse. Dorsal spines absent.] 

 Spathobatis, Rhinobatus. Range, Jurassic — Recent. 



19. ToRiEDiNiD^E. [Body naked, much depressed, and 

 gently rounded in front. Provided with an electric organ.] 

 Cyclobatis. Range, Cretaceous — Recent. 



20. Rajid.e. ["Rays" proper. Body generally protected 

 with rows of spinous dermal tubercles, which are often found 

 fossil.] Raja. Range, Pliocene — Recent. 



21. TRYGONID.E. ["Sting-Rays." Teeth usually flattened. 

 Tail often armed with barbed spine.] Trygon, Urolophas. 

 Range, Eocene — Recent. 



22. ^IvLIOBATID.e. ["Eagle-Rays." Often attain to a large 

 size, and sometimes live in open sea far from the coast. Teeth 

 flattened, and forming a kind of tessellated pavement in both 

 jaws. Occasionally a barbed spine behind the dorsal fin.] 

 Myliobatis, yEtobatis, Zygobatis. Range, Eocene— Recent. 



GOSSIP ON CURRENT TOPICS. 



NOT long ago the geological conundrum of the 

 origin of the chalk appeared to be answered ; 

 "the continuity of the chalk" was an established 

 phrase, but its endurance was very brief. The idea 

 that our characteristic national sea-walls are the 

 ancient representatives of the globigerina ooze, still 

 in course of deposition, and that there has been no 

 Ijreak of time between, appears now to be officially 

 rej^laced by the statement that" chalk "must be re- 

 garded as having been laid down rather along the 

 border of a continent than in a true oceanic area." 



It is rather perplexing that these opposite induc- 

 tions are both based on the results of the great 

 national yachting expeditions. Once upon a time, 

 before " The General Results of the Dredging CruLses 

 of H.M..S..S. Porcupine and Lightning," were 

 published " under the scientific direction of Dr. 

 Carpenter," &c., I could spend a holiday on the 

 south coast, bask on the l)each, and peacefully con- 

 template the chalk cliffs and their horizontal lines of 

 flint nodules without suffering any brain-splitting per- 

 plexities concerning their origin. Somehow and 

 somewhere I had imbibed a very simple theory ctm- 

 cerning the formation of chalk. I supposed it to be 

 a redeposited limestone ; that an older rock, such as 

 the mountain limestone of the carboniferous period, 

 had been exposed over a large area and denuded ; 

 that this denudation had formed a calcareous mud in 

 the sea, which ultimately consolidated ; that sponges 

 abounded there, and as they died and their sarcode 

 decomposed, their spicules were rolled over and 

 aggregated into rounded lumps by tidal or other 

 currents, and thus the flints were formed. Whether 

 this theorywas derived from Jamieson's lectures, which 

 supplied my first geological notions, when both 

 myself and geology were young, or whether from 

 some old geological treatise, or whether I dreamed it 

 while dozing on the beach, I know net ; but after 

 struggling through Wyville Thompson's book on the 

 above-named expeditions, and the more recent 

 contradictions supplied by the Challenger expedition, I 

 am strongly disposed to revert to the old notions and 

 look upon those cliffs again as peacefully as of yore. 



The absence of meteoric particles of glassy spicules 



