NO. 3683 CIRRIPEDIA—ROSS 6 
there apparently has been no further attempt to reevaluate the 
systematic position of these barnacles. Assignment of the tetraclitids 
as a subfamily of the Balanidae seems untenable at this time because 
they obviously represent a different and distinct lineage that in many 
respects parallels closely that of the Chthamalidae. 
The tetraclitids are compounded of forms with a shell consisting 
of only four tubiferous plates, the number of tubes ranging from one 
row in Tesseropora to several rows in Tetraclita and Tetraclitella. 
In the latter group the radii are also tubiferous, and only in this 
group are the radi as well developed as they are in the Balanidae. 
The basis with few exceptions is membraneous, and where calcareous, 
it is neither tubiferous nor does it possess complex ridges marginally 
that interdigitate with the basal margin of the wall. 
One of the salient structures of primary importance in the classi- 
fication of the Balanomorpha is the state of development of the 
labrum. In the Chthamalidae there is no notch in the labrum, and 
it is effectively bullate, whereas in the Balanidae the labrum is dis- 
tinctly notched and not bullate. In the tetraclitids this structure is 
clearly not bullate, nor is it notched, but rather it holds an inter- 
mediate position since it is thin, as in the balanids, and the crest 
has a saddle-like groove, as in Chthamalus. The presence of a bullate 
or simple un-notched labrum indicates a primitive evolutionary state, 
which is readily recognizable in the chthamalids and, of the other 
Thoracica, in the verrucomorphs and lepadomorphs. On the basis 
of the foregoing evidence alone it is not likely that an un-notched 
labrum could have evolved from a notched labrum; hence, the tetra- 
clitids could not have evolved from the balanids. 
Modifications in tetraclitid mandibular structure closely parallel 
that found in the chthamalids, wherein there are groups with either 
a comblike or serrate inferior margin. The inferior margin is never 
molariform as is frequently the case in the Balanidae. 
The first three pairs of cirri in the Balanidae are highly modified 
and effectively serve as mouth appendages. In Chthamalus and related 
genera only the first two pairs of cirri are so modified. In the tetra- 
clitids, cirrus III is modified as a mouth appendage but significantly 
less so than in the Balanidae. In the Balanidae the third cirrus is 
never antenniform, but it commonly is in the Chthamalidae and 
rarely in tetraclitids of the Tetraclita squamosa complex. 
Other features that argue for exclusion of the tetraclitids from the 
Balanidae are the absence of a basidorsal point on the intromittant 
organ and the presence of comb setae on the anterior cirri. Caudal 
appendages, present in many of the chthamalids, are lacking in the 
tetraclitids as well as the balanids. 
