108 The Rev. W. D. Conybeare's Additional Notices 



the teeth of the ichthyosaurus is, that in this animal the succession of new sets 

 of teeth was but seldom repeated^ and perhaps not more than once. 



II. Determination of the different Species of Ichthyosaurus from 

 THEIR Teeth. — When we consider the different forms which the same bones 

 are known to receive in the same animals at different ages, it is not possible to 

 approach without hesitation so delicate a problem, as that of determining, from 

 the teeth alone, the species of an animal, our knowledge of which depends 

 entirely upon fossil remains, often mutilated and obscure. Mr. De la Beche, 

 however, long since believed himself able, from the examination of the teeth, 

 combined with some other characters, to establish three species, to which he 

 has applied the names communis^ platj/odon, and tenuirostris : and to these 

 our joint observations have recently added a fourth. Ichthyosaurus interme- 

 dins. All these occur in the formation of lias. The ichthyosaurus whose 

 remains are found in the Kimmeridge clay, appears to belong to yet another 

 species : but our knowledge of its structure is not yet sufficiently precise, to 

 enable us to describe it. We have not seen its teeth; and its distinctive cha- 

 racters depend, at present, on its cervical vertebrae. 



The distinctions of teeth in the first four species above enumerated are as 

 follows : (see PI. XV.) 



1. I. communis, (fig. 8.) Upper part of the tooth conical, not very acute, 

 slightly aduncate, and thickly covered with prominent longitudinal striae. 



2. I. platyodon. (fig. 7.) Upper part of the tooth smooth and flattened, so 

 as to present sharpened edges. (See transverse section at c. fig. 7.) 



3. I. tenuirostris. (fig. 10.) In this the teeth are much more slender than in 

 the preceding species. But the species is best marked by the extreme length 

 and thinness of the snout, in which points it very strikingly exceeds all the 

 other ichthyosauri. 



4. I. intermedins, (fig. 9.) The upper part of the teeth is much more 

 acutely conical than in I. communis, and the striae less prominent : yet they 

 are less slender than in I. tenuirostris. This species is also distinguished by 

 differences (presently to be noticed) in the angular and coronoid bones of the 

 lower jaw. 



The specimens of I. platyodon are generally large : the most gigantic yet 

 discovered are referable to this species. The specimens of I. communis 

 occur of very different sizes ; such as may have belonged to animals from five 

 to fifteen feet in length when entire. Those of I. tenuirostris and I. inter- 

 medins appear not to exceed one half the largest size of I. communis : — these 

 last ob.servations, however, are offered with much diffidence, as it must be ob- 



