8o 



NATURE 



{Nov. 30, 1 871 



valves are dorsal and ventral, like the two parts of a 

 cabriolet in relation to the animal within, instead of lateral 

 (wing-like) as in Lamcllibranchs. Valves joined by hinge 

 or not ; never with elastic spring. When not hinged, the 

 valves imperforate ; when hinged, one, the larger, is 

 perforate for the transmission of an anchoring ligament, 

 in the non-hinged the ligament passes out between the 

 valves. The class is divided into two orders or subclasses, 

 — the Articulate and the Inarticulate. The Articulate, of 

 which TerebratuK is type, have usually curious shelly 

 processes developed from the inner surface of the imper- 

 forate valve for the support of the arms, and have in the 

 adult condition no anus ; the Inarticulate, of which 

 Lingula is type, have no arm-supporting processes and 

 have no anus." 



The account given of the vertebrate skeleton, and 

 especially of some disputed questions of homology, is not 

 so satisfactory as most other parts of the Syllabus. It 

 may be doubtful whether it is desirable to introduce into 

 elementary lectures the difficult subject of the representa- 

 tives of the tympanic bones in the lower vertebrata ; but if 

 so, it is quite useless for men to learn to repeat the " views " 

 of Owen, Huxley, Peters, Parker, and Humphry, and to 

 assign the right view to the right man, unless they are 

 familiar with the facts of embryology, on which alone a 

 judgment can be formed. Now, whether the incus belongs 

 to the first visceral arch, as here stated (p. 113), or to the 

 second, as is believed by some original observers, makes 

 all the difference as to the correctness or incorrectness of 

 the statements which follow. Again, whatever doubt still 

 remains as to the homologies of the pelvis and shoulder 

 girdle, surely no one who has read Prof. Flower's paper 

 on the subject and his subsequent remarks in the " Osteo 

 logy of the Mammalia," can accept the correspondence 

 of the pubcs with the clavicle. The former may very 

 probably answer to a procoracoid, as Gegenbaur and 

 other anatomists suppose, but its mode of development 

 its position in reptiles, and its relation to the great 

 nerves and vessels of the hind limb, are all conclusive 

 against the homology given in p. 116, and more fully in 

 p. 146. No reason is assigned for the query affixed to the 

 statement (p. 171) that the elephant's placenta is deciduous 

 and zonary, which zoologists have hitherto accepted on 

 the testimony of more than one careful and independent 

 observer. The statement as to the number of the cervical 

 vertebnc in mammalia (p. 172) is not exact. No Cetacean 

 has yet been found in which the full number cannot be 

 distinguished, however much fuced together the vertebra; 

 may become. On the other hand, the manati has never 

 more than six, and the same appears to be true of one 

 species of Cliolopus (not Cholcepus). 



No mention is made of the order Dipnoi in the classifi- 

 cation of fishes taken from Miiller (p. 117), or again in 

 the characters of the orders (pp. 133-135). -So remarkable a 

 form as Lepidosircn should not have been omitted, even if 

 Dr. Ord accepts the conclusion which Dr. Giinther has very 

 lately stated in these columns (vol. iv. Nos. 99 and 100). 

 The new genus Ccratodiis, now that its anatomy has been 

 so fully investigated, forms no doubt a very complete link 

 between the Ganoids and the Dipnoi, and many zoologists 

 will agree with the classification proposed in the ad- 

 mirable paper just referred to ; but books intended for 

 students should scarcely pursue the " latest views " so 

 closely. 



In conclusion it is only fair to repeat that these Notes 



deserve commendation for their geneial accuracy, and 

 contrast very favourably with sout; other manuals for 

 students on the same subject. They wil!, if well used, be 

 valuable to learners, and perhaps still more so to teachers. 

 P. H. Pye-Smith 



OUR BOOK SHELF 



Note-book on Practical Solid or Descriptive Geometry, 

 containing Problems with help for Solutions. By J. H. 

 Edgar, M.A., Lecturer on Mechanical Drawing at 

 the Royal School of Mines, &c., &c., and G. S. Prit- 

 chard, late Master for Descriptive Geometry, Royal 

 Military Academy, Woolwich. (London and New York : 

 Macmillan and Co., 1871.) 

 When our Civil and Military Engineering Examinations 

 are daily making larger demands for geometrical profi- 

 ciency a new and exceedingly lucid Note-book on 

 Descriptive Geometry comes well-timed. Though much 

 has been done to expand this collateral offshoot of 

 geometricalscience since M.Monge, of theEcole Polytech- 

 nique, first started it, the co-ordinative characteristic of a 

 science has hitherto been wanting ; it has contained, 

 doubtlessly, all the abstract principles of orthographic 

 projection, but principles, to be .available, must be inter- 

 dependent and derivative. Messrs. Edgar and Pritchard 

 have felt this deficiency, and have done much to remove 

 it. Their book, unlike the majority of cheap hand-books, 

 is neither "patchy nor scrappy," but a continuous and 

 coherent whole. " Elementary Explanations, Definitions, 

 and Theorems " come first, followed by twenty-eight pro- 

 blems on '' The Straight Line and Plane ;" to these suc- 

 ceed Solids, first singly, and then in " Groups and 

 Combinations." In like logical order we next have 

 '■ .Solids with the inclinations of the plane of one 

 face, and of one edge or line in that face given," 

 and then " Solids with the inclinations of two adjacent 

 edges given," and, lastly, in this category, " Solids with 

 the inclinations of two adjacent faces given." So far we 

 have the principles of projection in a much more per- 

 fectly co-ordinated arrangement than we have hitherto 

 found them in, and we must say that the mere act of 

 mentally assimilating this interdependence of principles 

 would be wholesome discipline, even if it did not, as it 

 unquestionably does, facilitate each successive step in 

 progress, and, most of all, conduce to an integral enter- 

 tainment of the subject. Again, as naturally derivable 

 from the consideration of the inclined faces of solids, we 

 arrive at " Sections by oblique planes," and " Develop- 

 ments," or the spreading out in one plane of the adjacent 

 faces of such solids ; and, finally, the development of 

 curved surfaces. " Miscellaneous Problems " now have 

 place, and amongst them we notice one from the " Science 

 Examinations " of last year. The sequence of the four 

 next chapters is judicious. " Tangent Planes," " Inter- 

 sections of solids with plane surfaces," " Intersections of 

 solids with curved surfaces," " Spherical Triangles." A 

 short chapter on Isometric Projection (quite as long as it 

 deserves) ends the work, the authors of which we rejoice 

 to find (in these days of " result-seeking ') much more 

 desirous of results actual than results visible, and ac- 

 cordingly, foregoing a somewhat too popular profusion of 

 diagrams, which, \\hile it undoubtedly facilitates the bare 

 apprehension of subject-matter, by no means enforces that 

 comprehension of the subject which attends upon the act 

 of accomplishing a mental diagram for ourselves. In this 

 expression of their conviction the authors, we observe, 

 are at one with Mr. Binns, who, with the same sincerity, 

 and for like reason, resisted the systematic use of models 

 in the teaching of "mechanical drawing." 



Messrs. Edgar and Pritchard have produced an inex- 

 pensive, but a well-digested, comprehensive, lucid, and 

 typographically attractive vade meciim. 



