ANIMAL TO ITS SHELL IN SOME FOSSIL CEPHjVLOPODA. 77 



Baculites, Lamarck. 



Baculites ovatus, Say. — An example of this species in the British Museum 

 (No. C. 5415 f/) from the Fox Hill beds (Upper Cretaceous) of Horse-head Creek, South 

 Dakota, U.S.A., displays the impression of the shell-muscles very clearly. The specimen 

 consists of the natural cast of nearly the whole of the body-chamber, the cast of the 

 last locvilus and of a portion of the penultimate loculus. The test has been almost 

 entii'ely removed from the posterior part of the body-chamber (PI. 17. figs. 1, 2, ifc 3). 

 The shell in this species is laterally compressed, and tapers very slowly. The length of 

 the specimen is 290 mm. or abont 11^ inches ; its transverse section is oval, the greatest 

 thickness being a little nearer the antisiphonal (dorsal) than the siphonal (ventral) 

 area. Of this length the body-chamber occupies 215 mm., but the aperture is not 

 preserved. The ventro-dorsal and transverse diameters of the base are 38 and 25'5 mm. 

 respectively. The two muscular impressions are at the base of the body-chamber and on 

 the antisij)honal (dorsal) area ; the inner, antei'ior, and outer portions of their botmdaries 

 can be distinctly traced as a faint, shallow, depressed (not sharply-incised) line. On the 

 antisiphonal area the suture-line has a small antisiphonal lobe separating the two portions 

 of a broad saddle, each portion corresponding to the second lateral saddle in an ammonite ; 

 each of these halves is followed by a rather broad lobe (the second lateral) which separates 

 it from the saddle occupying nearly the middle of the lateral area, L e. the first lateral 

 saddle. The median line of the antisiphonal area is occupied by a very shallow longitu- 

 dinal groove. The inner boundaries of the two muscvilar impressions seem to meet in 

 the middle line at about 2'5 mm. in advance of the most anterior part of the saddle, 

 adjoining the antisiphonal lobe. Starting from this point, the boundary of the impression 

 passes forward and outward until it is S"5 mm. from the same j)art of the suture-line ; 

 then, turning backward and still maintaining its course outward, it passes along the 

 outer side of the second lateral lobe, close to the inner side of the first lateral saddle. 

 The boundary of the other impression has a precisely similar course. The impressions 

 are somewhat oval, their longer diameters making an angle of about 45 with the median 

 line of the antisiphonal (or dorsal) area. At a point slightly below the level of 

 the most anterior part of the adjoining saddle (the first lateral) the outer boundary of 

 each impi'cssion is a little angular. On one (the right) side of the specimen no trace of 

 the annulus can be seen, but on the other (the left) side a line is seen to pass from the 

 angular portion of the impression outward and upw'ard over, and about 1 mm. distant 

 from, the adjoining saddle ; although it can be traced only for a short distance, owing to 

 the roughness of the siuface here, it doubtless represents the anterior boundary of a 

 portion of the annulus. 



At a distance of 5 mm. in front of the boundary of the impression on the right side, 

 there is another line having precisely the same curvature. It is not nearly so distinct as 

 the one just described, but most probably indicates the anterior boundary of the same 

 shell-muscle. Possibly the posterior line denotes the position of the shell-muscle diu'ing 

 the formation of a septum, and therefore dui'ing a period of rest, when the muscle would 



