Roman unit: Celtae Seniores

Celtae Seniores were a auxilia palatina unit under the command of the Italian Magister Peditum

Details of their shields pattern are reported into the Notitia Dignitatum (Oxford version) and it is possible to see them below:

(the fourth of the third line starting from left)

Here the free links to Notitia Dignitatum resources:

Oxford version:
http://bodley30.bodley.ox.ac.uk:8180/luna/servlet/view/all/what/MS.+Canon.+Misc.+378?sort=Shelfmark%2cFolio_Page%2cRoll_%23%2cFrame_%23

Paris version:

http://gallica.bnf.fr/ark:/12148/btv1b6000542r.r=latin+9661.langEN

Munich version:

http://daten.digitale-sammlungen.de/~db/bsb00005863/images/index.html?fip=193.174.98.30&seite=179&pdfseitex

Their shield shows a very common theme for late roman units (in particular for auxilia palatina units) that is a kind of twin-headed zoomorphic motif.

The name  "Celtae" refers obviously to "Celts"

It is possible to find references to Celtae in Ammianus. During the Julian wars against the Alemannics they are generally linked to another auxilia palatina unit: the petulantes. 

It possible that they took part to Strasburg battle (357 AD) [Ammianus XVI, 12]. It is sure that they are part of Julian army after his appointment to Emperor [Ammianus XX.5] 

Below a photo in which  a group of Celtae Seniores are attacked by an Alemannic tribe (the Lentienses Alamanni)[Ammianus XXI.10.4] 

If someone is interested to the Roman army of the late Empire  it is possible to find more details in these books:
- A companion to the Roman Army - Paul ErdKamp - Blackwell Publishing 
- The grand strategy of the Roman Empire: from the first century AD to the third - Edward Luttwak
- L'esercito romano vol III - Giuseppe Cascarino e Carlo Sansilvestri - Ed Il cerchio
- L'esercito romano vol IV - Giuseppe Cascarino e Carlo Sansilvestri - Ed Il cerchio
- L'esercito romano: da Augusto alla fine del terzo secolo - Yann Le Bohec - Ed Carocci
- 9 agosto 378. Il giorno dei barbari - Alessandro Barbero - Ed Laterza
- Le Storie - Ammiano Marcellino - Ed Utet
- L'arte della guerra - Flavio Vegezio - Ed Bur


and many Osprey Publishing books.