10 MB. E. LYDEKXER ON THE CETACEA OE THE STJEEOLK CRAG. 



similar to those of the type form. This very remarkable type of 

 tympanic presents an approximation to the still more remarkable 

 tympanic of the genus Neobalcena, the resemblance being so decided 

 as to indicate the probability of there having been a genetic connec- 

 tion between the two forms. The form which I note as variety C 

 is represented by a tympanic in the British Museum (iso. 46685), 

 which, while agreeing in many respects with the type-form, is 

 distinguished by its extreme lateral compression and the total 

 absence of any thickening of the involucrum. 



The last variety, which we may term D, is characterized by 

 the lowness of the inner wall, the parallelism of its superior border 

 to the long axis of the bone, the slight downward extent of the 

 flattening of the anterior surface, and the absence of any distinct 

 angulation of the inferior border. There is a fine example of this 

 form in the British Museum (PI. II. figs. 2, 2a), and a smaller one 

 in the Museum of Practical Geology. It is not always easy to dis- 

 tinguish between some of the smaller tympanies of this type and 

 those of B. insignis ; but those of the latter are usually more inflated, 

 thicker inferiorly, and with a distinct angulation of the inferior 

 border. Tympanies agreeing with those of B. (Balcenotus) insignis 

 and B. (Bcdcenula) balcmopsis, Van Beneclen, occur in the Bed Crag ; 

 and the British Museum possesses an atlas vertebra (No. M. 3542) 

 from the Coralline Crag referred Joy Professor Van Beneden' to the 

 latter species. Both these Whales are of* small size, but B. balcen- 

 opsis is the smaller of the two. Prof. Van Beneden mentions 

 certain structural differences by which the tympanies of these two 

 species can be distinguished ; but I confess that I was unable to 

 satisfy myself of the validity of such distinctions from an exami- 

 nation of the type specimens in the Brussels Museum : in naming 

 the Crag specimens, I have been forced to content myself with 

 referring the smaller ones to B. balcenojpsis, and the larger to B. in- 

 signis ; and it is difficult, in the case of some immature specimens, 

 to say whether they may not belong to young individuals of the 

 larger species. There is a very beautiful example of the right tym- 

 panic of the former species, which was obtained from the Bed Crag 

 of Woodbridge, preserved in the Ipswich Museum. The tympanic 

 of B. insignis frequently exhibits a flattening of the inferior surface 

 which recalls the structure obtaining in the following section. 



In the Balaenopterine section, which is characterized by the absence 

 of anchylosis of the cervical vertebras and the more inflated and 

 rounded tympanic, in which the Eustachian channel is usually well 

 defined, it may be observed that a very large nnmber of genera and 

 species have been founded by Prof. Van Beneden on the evidence 

 of remains from the Antwerp Crag. Some of these forms have not 

 yet been figured, and their names are therefore little better than 

 MS. ones ; while in those which have been fully described it appears 

 to me that some of the generic divisions are unnecessary, and I 

 cannot help thinking that in certain instances some of the forms to 

 which specific names have been applied are not improbably only 

 individual or sexual variations ; but as it is impossible to prove this, 



