164 J. WILFRID JACKSON ON 



Both specimens (PI. II. fig. 10) exhibit a terebratelliform condition in the 

 brachial support. The descending branches, which extend two-thirds the length of 

 the valve, are very thin and are attached to the median septum by a slender jugal 

 band nearly at right angles with the descending branches. 



The ascending branches follow much the same course as those below, and are united 

 by means of a short transverse band which is slightly inflected in its median portion.* 



The crural points are very short and not turned inwards towards each other, as in 

 Magellania venom and Terebratella dorsata, but are directed upwards in the direction 

 of the ventral valve. The jugal band is situated about the middle of the length of the 

 loop, and is fixed to the terminal part of the septum ; it is quite as slender as the other 

 portions of the apparatus. 



It would appear from certain indications on the descending branches that the 

 metamorphosis of the loop is still uncompleted, as the inside edges of these, at their 

 junction with the jugal band, exhibit traces of an oblique suture, a feature hitherto not 

 observed in adult Terebratella dorsata. 



The umbonal cavity of this valve is occupied by a well-developed cardinal plateau 

 fixed to the bottom of the valve, and depressed longitudinally in its median part in the 

 form of a trough, from the anterior end of which extends a thin-edged median septum. 

 The posterior extremity of the plateau carries a well-developed transverse cardinal pro- 

 cess ; the lateral parts form two somewhat triangular plates bordering the dental sockets. 



Externally the shell is of an oval form, longer than broad, attaining its maximum 

 diameter about the middle of the valves (PI. II. fig. 10). Line of joining slightly 

 flexuous at the frontal and lateral margins. Valves swollen, the ventral being deeper 

 than the dorsal. Surface roughened by numerous well-pronounced growth-lines, which 

 are close set. Specimen No. 2 exhibits very numerous close-set growth-lines at the 

 margins similar to those seen in specimens which have attained a senile condition. 



The test is very solid and opaque. Beak of ventral valve moderately produced, 

 incurved, and truncated by a large foramen, with rudimentary deltidial plates. Sides 

 of the beak carinated, forming a flattened area below the foramen. The test is covered 

 with very numerous perforations, but owing to the difficulty in lighting I have been 

 unable to count them satisfactorily under the microscope. 



Owing to the want of further material the exact identification of this interesting 

 form is extremely difficult. It can scarcely be regarded as a terebratelliform stage 

 of Magellania venosa, as at this stage in its ontogeny the latter species is much 

 more transverse and the branches of the loop broader (see especially FISCHER and 

 OEHLERT, 1892, pi. xi. fig. 8). 



Neither can it be looked upon as a small adult Terebratella dorsata, on account of 

 the entire absence of the characteristic surface sculpture of that species, as well as the 

 distinct difference in shape and the discordance in the composition of the brachial 

 support. 



* These are broken off in photograph. 

 (ROY. soc. EDIN. TRANS., VOL. XLVIII., 386.) 



