46 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
bearing bracliiopods the t^'pes of jugum are considered in much the 
same manner as in the following extract from Hall and Clarke ('92, p. 
203-205) : 
"The jugum does not end at the point of union of the two lateral 
jugal processes [or lateral branches], but is more or less continued, in 
its simplest form, into a short, straight stem, inclined backward and 
terminating in an acute angle (Retzia, Rhynchospira, Whitfieldella) ; 
in Bifida this stem is long and extends to the inner surface of the pedicle- 
valve where its extremity is inserted into a deep groove in the shell. 
In Eumetria and Meristina the extremity of the stem is bifurcate, but 
the terminal processes do not extend beyond the ends of the crura; 
while in Athyris, Tetractinella and Amphitomella they are continued 
backward between the primary and secondary lamellae for about one 
fourth of a revolution. In Pexidella the intercalary lamellae are still 
longer; in Amphitomella,^ Kayseria and Amphiclina they are con- 
tinued for the entire length of the spiral cones thus making a double 
spiral on each side. In Merista and INIeristella the divisions of the 
stem of the jugum, or intercalary lamellae, are short, recurving and 
joining the lateral processes near their union." 
In this and similar descriptions, the stem is regarded as a continua- 
tion of the united jugal processes or lateral branches, and elsewhere 
the series of gradual stages which is illustrated by the genera named, 
is regarded by the same authors ('95, p. 345) as indicating in a general 
way the genetic line that has been followed in the production of such 
complicated juga as are seen in Pexidella, Diplospirella, Kayseria, 
etc. The jugum is believed to have been evolved step by step; first 
a simple A-shaped bar between the primary lamellae, to the apex of 
which a simple stem was later added ; in time the free end of this stem 
became slightly cleft, the two lamellae thus formed, lengthening and 
passing to the outside of the primary lamellae and recurving, more or 
less parallel to them. Two general lines of development were now 
followed: the lamellae formed from the split stem might unite with 
the base of the stem at or just above the apex of the A as in the Meristel- 
linae, or they might continue in their process of elongation parallel 
to the ribbons of the coil until a double spiral was formed on either 
side. Hall and Clarke suggest that the last described type may have 
been developed independently in three entirely separate fines of descent, 
1 The use of Amphitomella is probably an error overlooked in the preparation of the 
work. 
