316 THE DOLABELLINAE. 



many of the finer terminations pass out of the plane of vision. In fig. 1, Plate 8, 

 two groups of muscle-fibres b, b, terminate in the same manner. Here also a 

 change of focus shows that the interlacing feltwork of the fine branches is even 

 more extensive than here shown. So far as this material permits conclusions 

 to be drawn there exists a continuity of the finest prolongations of the muscle- 

 fibres with the epithelium-cells. An actual continuity of the myofibrillae with 

 epithelial fibrillae, such as described by Briick (1813) for Anodonta and Cyclas, 

 and by many other writers for arthropods, could not be satisfactorily established, 

 though there is much to favor its existence. 



Radula. — Immediately behind the mandibles Ues the large and strong 

 deep amber radula. In situ its exposed surface presents a conventional heart- 

 shape, as seen from above, the anterior narrower end being abruptly rounded 

 and continued downward into a deep groove in front and laterally. The dorsal 

 groove of the radula is deepest behind, and extends forward two thirds the 

 length of the organ, becoming shallower in front, while behind it merges into 

 a transverse groove. The highest portions of the radula thus form the sides 

 of the median groove, and round over toward the sides and more abruptly in 

 front. The rows of functional teeth curve around over this elevated sm-face, 

 in general parallel to its external contour, Plate 8, fig. 5. Deep in the median 

 groove the youngest teeth are covered with the epithelium of the sheath. When 

 freed from its attachments, cleaned, and flattened out, the shape of the radula 

 is approximately rectangular, the sides being nearly parallel, the posterior 

 border nearly straight, while the anterior margin is bluntly pointed. In extreme 

 width it measures ca. 18.4 mm., in median length, 14.3 mm., while the sides 

 are 11.8 mm. long. 



The teeth are arranged in sixty-two transverse rows, the first four of which 

 are incomplete, worn, and broken. Succeeding these are thirty-six rows which 

 are exposed and functional, and then twenty-four rows which are still inclosed 

 within the radula-sheath. The number of teeth in each half row varies, increas- 

 ing from before backward from 170 in the eighth, and 198 in the twenty fifth 

 to a maximum of 230 in the forty second row. The dental formula of Dola- 

 bella agassizi may hence be given as 62 X 198-230- 1-198-230, there being a 

 single median tooth in each row, and the total number of teeth probably being 

 not far from 25,p00. 



The median tooth is but Uttle more than an elongated slightly flattened 

 rudiment in the first twelve rows of the radula (Plate 6, fig. 3, 4). Further 

 back it develops a transverse club-like anterior thickening, the tip being curved 



