265. A section of the shell of _Melo ethiopicus_ . . . 525 266. Shells of _Harpa_ and _Dolium_, to illustrate generating curves and gene . . . 526 267. D’Orbigny’s Helicometer . . . 529 268. Section of a nautiloid shell, to shew the “protoconch” . . . 531 269–73. Diagrams of logarithmic spirals, of various angles . . . 532–5 274, 6, 7. Constructions for determining the angle of a logarithmic spiral . . . 537, 8 275. An ammonite, to shew its corrugated surface pattern . . . 537 278–80. Illustrations of the “angle of retardation” . . . 542–4 281. A shell of _Macroscaphites_, to shew change of curvature . . . 550 282. Construction for determining the length of the coiled spire . . . 551 283. Section of the shell of _Triton corrugatus_ (Woodward) . . . 554 284. _Lamellaria perspicua_ and _Sigaretus haliotoides_ (_do._) . . . 555 285, 6. Sections of the shells of _Terebra maculata_ and _Trochus niloticus_ . . . 559, 60 287–9. Diagrams illustrating the lines of growth on a lamellibranch shell . . . 563–5 290. _Caprinella adversa_ (Woodward) . . . 567 291. Section of the shell of _Productus_ (Woods) . . . 567 292. The “skeletal loop” of _Terebratula_ (_do._) . . . 568 293, 4. The spiral arms of _Spirifer_ and of _Atrypa_ (_do._) . . . 569 295–7. Shells of _Cleodora_, _Hyalaea_ and other pteropods (Boas) . . . 570, 1 298, 9. Coordinate diagrams of the shell-outline in certain pteropods . . . 572, 3 300. Development of the shell of _Hyalaea tridentata_ (Tesch) . . . 573 301. Pteropod shells, of _Cleodora_ and _Hyalaea_, viewed from the side (Boas) . . . 575 302, 3. Diagrams of septa in a conical shell . . . 579 304. A section of _Nautilus_, shewing the logarithmic spirals of the septa to which the shell-spiral is the evolute . . . 581 305. Cast of the interior of the shell of _Nautilus_, to shew the contours of the septa at their junction with the shell-wall . . . 582 306. _Ammonites Sowerbyi_, to shew septal outlines (Zittel, after Steinmann and Döderlein) . . . 584 307. Suture-line of _Pinacoceras_ (Zittel, after Hauer) . . . 584 308. Shells of _Hastigerina_, to shew the “mouth” (Brady) . . . 588 309. _Nummulina antiquior_ (V. von Möller) . . . 591 310. _Cornuspira foliacea_ and _Operculina complanata_ (Brady) . . . 594 311. _Miliolina pulchella_ and _linnaeana_ (Brady) . . . 596 312, 3. _Cyclammina cancellata_ (_do._), and diagrammatic figure of the same . . . 596, 7 314. _Orbulina universa_ (Brady) . . . 598 315. _Cristellaria reniformis_ (_do._) . . . 600 316. _Discorbina bertheloti_ (_do._) . . . 603 317. _Textularia trochus_ and _concava_ (_do._) . . . 604 318. Diagrammatic figure of a ram’s horns (Sir V. Brooke) . . . 615 319. Head of an Arabian wild goat (Sclater) . . . 616 320. Head of _Ovis Ammon_, shewing St Venant’s curves . . . 621 321. St Venant’s diagram of a triangular prism under torsion (Thomson and Tait) . . . 623