The largest known species of _Ichthyosaurus_ is _I. trigonodon_ from the Upper Lias of Banz, Bavaria, with the head measuring about two metres in length and probably representing an animal not less than ten metres in total length. _I. platyodon_, from the English Lower Lias, seems to have been almost equally large. _I. intermedius_ and _I. communis_, which are the commonest species in the English Lower Lias, rarely exceed a length of three or four metres. The species in rocks later than the Lias are known for the most part only by fragments, but the remains of Lower Cretaceous age are noteworthy for their very wide geographical distribution, having been found in Europe, the East Indies, Australia, New Zealand and South America. Allied Ichthyosaurians named _Ophthalmosaurus_ and _Baptanodon_, from the Upper Jurassic of England and North America, are nearly or quite toothless and have very flexible broad paddles. The earliest known Ichthyosaurians (_Mixosaurus_), which occur in the Trias, are of diminutive size, with paddles which suggest that these marine reptiles were originally descended from land or marsh animals (see REPTILES). AUTHORITIES.--R. Owen, _A Monograph of the Fossil Reptilia of the Liassic Formations_, part iii. (Mon. Palaeont. Soc., 1881); E. Fraas, _Die Ichthyosaurier der süddeutschen Trias- und Jura-Ablagerungen_ (Tübingen, 1891). Also good figures in T. Hawkins, _The Book of the Great Sea-dragons_ (London, 1840). (A. S. Wo.) ICHTHYOSIS, or XERODERMA, a general thickening of the whole skin and marked accumulation of the epidermic elements, with atrophy of the sebaceous glands, giving rise to a hard, dry, scaly condition, whence the names, from [Greek: ichthys], fish, and [Greek: xêros], dry, [Greek: derma], skin. This disease generally first appears in infancy, and is probably congenital. It differs in intensity and in distribution, and is generally little amenable to any but palliative remedies, such as the regular application of oily substances. Ichthyosis lingualis ("smokers' tongue"), a variety common in heavy smokers, occurs in opaque white patches on the tongue, gums and roof of the mouth. Cancer occasionally starts from the patches. The affection is obstinate, but may disappear spontaneously.